Observation of interference and diffraction of light laboratory conclusion. Photo report “Observation of interference and diffraction of light at home

Laboratory work No. 11. Observation of the phenomenon of interference and diffraction of light.
The purpose of the work: to experimentally study the phenomenon of interference and diffraction of light, to identify the conditions for the occurrence of these phenomena and the nature of the distribution of light energy in space.
Equipment: an electric lamp with a straight filament (one per class), two glass plates, a PVC tube, a glass with a soap solution, a wire ring with a handle with a diameter of 30 mm, a blade, a strip of paper ¼ sheet, nylon fabric 5x5 cm, a diffraction grating, light filters .

Brief theory
Interference and diffraction are phenomena characteristic of waves of any nature: mechanical, electromagnetic. Wave interference is the addition of two (or several) waves in space, in which at its different points an amplification or weakening of the resulting wave is obtained. Interference is observed when waves are superimposed, emitted by the same light source, which came to a given point in different ways. For the formation of a stable interference pattern, coherent waves are needed - waves that have the same frequency and a constant phase difference. Coherent waves can be obtained on thin films of oxides, fat, on an air wedge-gap between two transparent glasses pressed against each other.
The amplitude of the resulting displacement at point C depends on the difference in the path of the waves at a distance d2 – d1.
[ Download the file to view the picture ] Maximum-(amplification of oscillations) condition: the difference in the path of the waves is equal to an even number of half-waves
where k=0; ± 1; ±2; ± 3;
[ Download the file to view the picture ] Waves from sources A and B will come to point C in the same phases and “amplify each other.
If the path difference is equal to an odd number of half-waves, then the waves will weaken each other and a minimum will be observed at the point of their meeting.

[ Download the file to view the image ][ Download the file to view the image ]
When light interferes, a spatial redistribution of the energy of light waves occurs.
Diffraction is the phenomenon of wave deviation from rectilinear propagation when passing through small holes and rounding small obstacles by the wave.
Diffraction is explained by the Huygens-Fresnel principle: each point of the obstacle reached by the wave becomes a source of secondary waves, coherent, which propagate beyond the edges of the obstacle and interfere with each other, forming a stable interference pattern - alternation of illumination maxima and minima, iridescently colored in white light. Condition for the manifestation of diffraction: The dimensions of the obstacles (holes) must be smaller than or commensurate with the wavelength. Diffraction is observed on thin filaments, scratches on glass, on a slit-vertical cut in a sheet of paper, on eyelashes, on water droplets on misted glass, on ice crystals in a cloud or on glass, on the bristles of the chitinous cover of insects, on bird feathers, on CDs, wrapping paper., On a diffraction grating.,
A diffraction grating is an optical device, which is a periodic structure of a large number of regularly arranged elements on which light is diffracted. Strokes with a profile defined and constant for a given diffraction grating are repeated through the same interval d (lattice period). The ability of a diffraction grating to decompose a beam of light incident on it into wavelengths is its main property. There are reflective and transparent diffraction gratings. In modern devices, mainly reflective diffraction gratings are used.

Progress:
Task 1. A) Observation of interference on a thin film:
Experience 1. Dip the wire ring in the soap solution. A soap film is formed on the wire ring.
Position it vertically. We observe light and dark horizontal stripes that change in width and color as the film thickness changes. Examine the picture through a light filter.
Write down how many bands are observed and how the colors alternate in them?
Experience 2. Using a PVC tube, blow a soap bubble and examine it carefully. When illuminated with white light, observe the formation of interference spots, painted in spectral colors. Examine the picture through a light filter.
What colors are visible in the bubble and how do they alternate from top to bottom?
B) Observation of interference on the air wedge:
Experience 3. Carefully wipe two glass plates, put together and squeeze with your fingers. Due to the non-ideality of the shape of the contacting surfaces, the thinnest air voids are formed between the plates - these are air wedges, interference occurs on them. When the force compressing the plates changes, the thickness of the air wedge changes, which leads to a change in the location and shape of the interference maxima and minima. Then examine the picture through a light filter.
Draw what you see in white light and what you see through a filter.

Conclude: Why interference occurs, how to explain the color of the maxima in the interference pattern, which affects the brightness and color of the picture.

Task 2. Observation of light diffraction.
Experience 4. With a blade we cut a slit in a sheet of paper, apply the paper to our eyes and look through the slit at the light source-lamp. We observe the maxima and minima of illumination. Then examine the picture through a light filter.
Sketch the diffraction pattern seen in white light and in monochromatic light.
Deforming the paper, we reduce the width of the slit, we observe diffraction.
Experience 5. Consider a light source-lamp through a diffraction grating.
How has the diffraction pattern changed?
Experience 6. Look through the nylon fabric at the thread of a luminous lamp. By turning the fabric around the axis, achieve a clear diffraction pattern in the form of two diffraction bands crossed at right angles.
Sketch the observed diffraction cross. Explain this phenomenon.
Make a conclusion: why diffraction occurs, how to explain the color of the maxima in the diffraction pattern, what affects the brightness and color of the picture.
Control questions:
What is common between the phenomenon of interference\erence and the phenomenon of diffraction?
What waves can give a stable interference pattern?
Why is there no interference pattern on the student table from lamps suspended from the ceiling in the classroom?

6. How to explain the colored circles around the moon?

Attached files

Laboratory work on the topic : "Observation of interference and diffraction of light"

Goal of the work: experimentally study the phenomenon of interference and diffraction.

Equipment: an electric lamp with a straight filament, two glass plates, a glass tube, a glass with a soap solution, a wire ring with a handle with a diameter of 30 mm, a CD, nylon fabric, a light filter.

Theory: Interference is a phenomenon characteristic of waves of any nature: mechanical, electromagnetic.

Wave interference – addition in space of two (or several) waves, in which at its different points an amplification or attenuation of the resulting wave is obtained .

Typically, interference is observed when the superposition of waves emitted by the same light source, which came to a given point in different ways. It is impossible to obtain an interference pattern from two independent sources, since molecules or atoms emit light in separate trains of waves, independently of each other. Atoms emit fragments of light waves (trains), in which the phases of oscillations are random. Tsugi are about 1 meter long. Wave trains of different atoms are superimposed on each other. The amplitude of the resulting oscillations changes chaotically with time so quickly that the eye does not have time to feel this change of pictures. Therefore, a person sees the space evenly lit. To form a stable interference pattern, coherent (matched) wave sources are needed.

coherent called waves that have the same frequency and a constant phase difference.

The amplitude of the resulting displacement at point C depends on the difference in the path of the waves at a distance d2 – d1.

Maximum condition

, (Δd=d 2 -d 1 )

Where k=0; ± 1; ±2; ± 3 ;…

(the difference in the path of the waves is equal to an even number of half-waves)

Waves from sources A and B will come to point C in the same phases and “amplify each other”.

φ A = φ B - oscillation phases

Δφ=0 - phase difference

A=2X max

Minimum condition

, (Δd=d 2 -d 1 )

Where k=0; ± 1; ±2; ± 3;…

(the difference in the path of the waves is equal to an odd number of half-waves)

Waves from sources A and B will come to point C in antiphase and "extinguish each other".

φ A ≠φ B - oscillation phases

Δφ=π - phase difference

A=0 is the amplitude of the resulting wave.

interference pattern – regular alternation of areas of high and low light intensity.

Light interference - spatial redistribution of the energy of light radiation when two or more light waves are superimposed.

Due to diffraction, the light deviates from a rectilinear propagation (for example, near the edges of obstacles).

Diffraction – the phenomenon of wave deviation from rectilinear propagation when passing through small holes and rounding small obstacles by the wave .

Diffraction manifestation condition : d< λ , Where d - the size of the obstacle,λ - wavelength. The dimensions of the obstacles (holes) must be smaller than or commensurate with the wavelength.

The existence of this phenomenon (diffraction) limits the scope of the laws of geometric optics and is the reason for the limiting resolution of optical instruments.

Diffraction grating - an optical device, which is a periodic structure of a large number of regularly arranged elements on which light is diffracted. Strokes with a profile defined and constant for a given diffraction grating are repeated at regular intervalsd (lattice period). The ability of a diffraction grating to decompose a beam of light incident on it into wavelengths is its main property. There are reflective and transparent diffraction gratings.In modern devices, mainly reflective diffraction gratings are used. .

The condition for observing the diffraction maximum :

d sinφ=k λ, Where k=0; ± 1; ±2; ± 3; d - grating period , φ - the angle at which the maxima are observed, and λ - wavelength.

From the maximum condition it followssinφ=(k λ)/d .

Let k=1, then sinφ kr =λ kr /d And sinφ f =λ f /d.

It is known that λ kr >λ f , hence sinφ kr >sinφ f . Because y=sinφ f - the function is increasing, thenφ kr >φ f

Therefore, the violet color in the diffraction spectrum is located closer to the center.

In the phenomena of interference and diffraction of light, the law of conservation of energy is observed . In the area of ​​interference, light energy is only redistributed without being converted into other types of energy. The increase in energy at some points of the interference pattern relative to the total light energy is compensated by its decrease at other points (total light energy is the light energy of two light beams from independent sources). Light stripes correspond to energy maxima, dark stripes correspond to energy minima.

Progress:

Experience 1. Dip the wire ring in the soap solution. A soap film is formed on the wire ring.

Position it vertically. We observe light and dark horizontal stripes that change in width as the film thickness changes.

Explanation. The appearance of light and dark bands is explained by the interference of light waves reflected from the film surface. triangle d = 2h.The difference in the path of light waves is equal to twice the thickness of the film. When placed vertically, the film has a wedge-shaped shape. The difference in the path of light waves in its upper part will be less than in its lower part. In those places of the film where the path difference is equal to an even number of half-waves, bright stripes are observed. And with an odd number of half-waves - dark stripes. The horizontal arrangement of the stripes is explained by the horizontal arrangement of lines of equal film thickness.

We illuminate the soap film with white light (from the lamp). We observe the coloration of light bands in spectral colors: at the top - blue, at the bottom - red.

Explanation. This coloration is explained by the dependence of the position of the light bands on the wavelength of the incident color.

We also observe that the bands, expanding and retaining their shape, move down.

If you use light filters and illuminate with monochromatic light, then the interference pattern changes (the alternation of dark and light bands changes)

Explanation. This is due to a decrease in film thickness, as the soap solution flows down under the action of gravity.

Experience 2. Blow a soap bubble with a glass tube and examine it carefully. When illuminated with white light, observe the formation of colored interference rings, colored in spectral colors. The top edge of each light ring is blue, the bottom is red. As the film thickness decreases, the rings, also expanding, slowly move down. Their annular shape is explained by the annular shape of lines of equal thickness.

Answer the questions:

Why are soap bubbles iridescent?

What shape are the rainbow stripes?

Why does the color of the bubble change all the time?

Experience 3 . Thoroughly wipe two glass plates, put together and squeeze with your fingers. Due to the non-ideal shape of the contacting surfaces, the thinnest air voids are formed between the plates.

Explanation: The surfaces of the plates cannot be perfectly even, so they touch only in a few places. Around these places, the thinnest air wedges of various shapes are formed, giving a picture of interference. In transmitted light, the maximum condition 2h=kl

Answer the questions:

Why are bright iridescent ring-shaped or irregularly shaped stripes observed at the points of contact of the plates?

Why does the shape and location of the interference fringes change with pressure?

Experience 4. Examine carefully from different angles the surface of the CD (which is being recorded).

Explanation : The brightness of the diffraction spectra depends on the frequency of the grooves deposited on the disk and on the angle of incidence of the rays. Almost parallel rays incident from the lamp filament are reflected from adjacent bulges between the grooves at points A and B. The rays reflected at an angle equal to the angle of incidence form an image of the lamp filament in the form of a white line. Rays reflected at other angles have a certain path difference, as a result of which the waves are added.

What are you observing? Explain the observed phenomena. Describe the interference pattern.

The surface of a CD is a spiral track with a pitch commensurate with the wavelength of visible light. On a fine-structured surface, diffraction and interference phenomena appear. The highlights of CDs are iridescent.

Experience 5. Look through the nylon fabric at the filament of a burning lamp. By turning the fabric around the axis, achieve a clear diffraction pattern in the form of two diffraction bands crossed at right angles.

Explanation : A white diffraction peak is visible in the center of the cross. At k=0, the wave path difference is equal to zero, so the central maximum is white. The cross is obtained because the threads of the fabric are two diffraction gratings folded together with mutually perpendicular slots. The appearance of spectral colors is explained by the fact that white light consists of waves of different lengths. The diffraction maximum of light for different wavelengths is obtained at different locations.

Sketch the observed diffraction cross. Explain the observed phenomena.

Experience 6.

Diffraction at a small hole

To observe such diffraction, we need a thick sheet of paper and a pin. Using a pin, make a small hole in the sheet. Then we bring the hole close to the eye and observe a bright light source. In this case, the diffraction of light is visible

Record the output. Indicate in which of your experiments the phenomenon of interference was observed, and in which diffraction . Give examples of interference and diffraction that you have encountered.

Control questions ( each student prepares answers to questions ):

What is light?

Who proved that light is an electromagnetic wave?

What is the speed of light in vacuum?

Who discovered the interference of light?

What explains the iridescent coloration of thin interference films?

Can light waves from two incandescent bulbs interfere? Why?

Why is a thick layer of oil not iridescent?

Does the position of the main diffraction maxima depend on the number of grating slits?

Why does the apparent iridescent color of a soap film change all the time?

The purpose of the lesson:

• generalize knowledge on the topic “Interference and diffraction of light”;
• continue the formation of experimental skills and abilities of students;
• apply theoretical knowledge to explain natural phenomena;
• promote the formation of interest in physics and the process of scientific knowledge;
• contribute to the expansion of the horizons of students, the development of the ability to draw conclusions from the results of the experiment.

Equipment:

• straight filament lamp (one per class);
• wire ring with a handle (works No. 1,2);
• a glass of soapy water (works No. 1,2);
• glass plates (40 x 60 mm), 2 pieces per set (work No. 3) (home-made equipment);
• caliper (work No. 4);
• nylon fabric (100 x 100 mm, home-made equipment, work No. 5);
• gramophone records (4 and 8 strokes per 1 mm, work No. 6);
• CDs (work No. 6);
• photographs of insects and birds (work No. 7).

Lesson progress

I. Actualization of knowledge on the topic “Light Interference” (repetition of the studied material).

Teacher: Before performing the experimental tasks, we will repeat the main material.

What phenomenon is called the phenomenon of interference?

Which waves are characterized by interference?

Define coherent waves.

Write down the conditions for interference maxima and minima.

Is the law of conservation of energy observed in interference phenomena?

Students (suggested answers):

– Interference is a phenomenon characteristic of waves of any nature: mechanical, electromagnetic. “Interference of waves is the addition in space of two (or several) waves, in which at its different points an amplification or weakening of the resulting wave is obtained.”

– For the formation of a stable interference pattern, coherent (matched) wave sources are needed.

- Coherent waves are waves that have the same frequency and a constant phase difference.

On the board, students write down the conditions for maximums and minimums.

The amplitude of the resulting displacement at point C depends on the difference in the path of the waves at a distance d 2 – d 1 .

figure 1 - maximum conditions	figure 2 - minimum conditions
, () where k=0; ± 1; ±2; ± 3;… (the difference in the path of the waves is equal to an even number of half-waves) Waves from sources S 1 and S 2 will come to point C in the same phases and “amplify each other”. Phases of oscillation Phase difference А=2Х max is the amplitude of the resulting wave.	, () where k=0; ± 1; ±2; ± 3;… (the difference in the path of the waves is equal to an odd number of half-waves) Waves from sources S 1 and S 2 will come to point C in antiphase and "extinguish each other". Phases of oscillation Phase difference A=0 is the amplitude of the resulting wave.

An interference pattern is a regular alternation of areas of increased and decreased light intensity.

- Interference of light - spatial redistribution of the energy of light radiation when two or more light waves are superimposed.

Consequently, in the phenomena of interference and diffraction of light, the law of conservation of energy is observed. In the area of ​​interference, light energy is only redistributed without being converted into other types of energy. The increase in energy at some points of the interference pattern relative to the total light energy is compensated by its decrease at other points (total light energy is the light energy of two light beams from independent sources).

Light stripes correspond to energy maxima, dark stripes correspond to energy minima.

Teacher: Let's move on to the practical part of the lesson.

Experimental work No. 1

“Observation of the phenomenon of light interference on a soap film”.

Equipment: glasses with a solution of soap, wire rings with a handle with a diameter of 30 mm. ( see figure 3)

Students observe interference in a darkened classroom on a flat soap film under monochromatic illumination.

On the wire ring we get a soap film and place it vertically.

We observe light and dark horizontal stripes that change in width as the film thickness changes ( see figure 4).

Explanation. The appearance of light and dark bands is explained by the interference of light waves reflected from the film surface. triangle d = 2h

The difference in the path of light waves is equal to twice the thickness of the film.

When placed vertically, the film has a wedge-shaped shape. The difference in the path of light waves in its upper part will be less than in its lower part. In those places of the film where the path difference is equal to an even number of half-waves, bright stripes are observed. And with an odd number of half-waves - light stripes. The horizontal arrangement of the stripes is explained by the horizontal arrangement of lines of equal film thickness.

4. Illuminate the soap film with white light (from the lamp).

5. We observe the coloration of light bands in spectral colors: at the top - blue, at the bottom - red.

Explanation. This coloration is explained by the dependence of the position of the light bands on the wavelength of the incident color.

6. We also observe that the strips, expanding and maintaining their shape, move down.

Explanation. This is due to a decrease in film thickness, as the soap solution flows down under the action of gravity.

Experimental work No. 2

"Observation of the interference of light on a soap bubble".

1. Students blow bubbles (See Figure 5).

2. We observe the formation of interference rings painted in spectral colors on its upper and lower parts. The top edge of each light ring is blue, the bottom is red. As the film thickness decreases, the rings, also expanding, slowly move down. Their annular shape is explained by the annular shape of lines of equal thickness.

Experimental work No. 3.

“Observation of the interference of light on an air film”

Students put clean glass plates together and squeeze them with their fingers (see Figure No. 6).

The plates are viewed in reflected light against a dark background.

We observe in some places bright iridescent ring-shaped or closed irregularly shaped stripes.

Change the pressure and observe the change in the location and shape of the stripes.

Teacher: Observations in this work are individual. Sketch the interference pattern you observe.

Explanation: The surfaces of the plates cannot be perfectly even, so they touch only in a few places. Around these places, the thinnest air wedges of various shapes are formed, giving a picture of interference. (picture No. 7).

In transmitted light, the maximum condition 2h=kl

Teacher: The phenomenon of interference and polarization in construction and engineering technology is used to study the stresses that arise in individual nodes of structures and machines. The research method is called photoelastic. For example, when the part model is deformed, the homogeneity of organic glass is violated. The nature of the interference pattern reflects the internal stresses in the part.(picture no. 8) .

II. Actualization of knowledge on the topic “Diffraction of light” (repetition of the studied material).

Teacher: Before doing the second part of the work, we will repeat the main material.

What phenomenon is called the phenomenon of diffraction?

Condition for the manifestation of diffraction.

Diffraction grating, its types and main properties.

Condition for observing the diffraction maximum.

Why is purple closer to the center of the interference pattern?

Students (suggested answers):

Diffraction is the phenomenon of wave deviation from rectilinear propagation when passing through small holes and rounding small obstacles by the wave.

Condition for the manifestation of diffraction: d < , Where d is the size of the obstacle, is the wavelength. The dimensions of the obstacles (holes) must be smaller than or commensurate with the wavelength. The existence of this phenomenon (diffraction) limits the scope of the laws of geometric optics and is the reason for the limiting resolution of optical instruments.

A diffraction grating is an optical device that is a periodic structure of a large number of regularly spaced elements on which light is diffracted. Strokes with a profile defined and constant for a given diffraction grating are repeated at regular intervals d(lattice period). The ability of a diffraction grating to decompose a beam of light incident on it into wavelengths is its main property. There are reflective and transparent diffraction gratings. In modern devices, mainly reflective diffraction gratings are used..

Condition for observing the diffraction maximum:

Experimental work No. 4.

“Observation of the diffraction of light by a narrow slit”

Equipment: (cm drawing no. 9)

1. We shift the slider of the caliper until a gap of 0.5 mm wide forms between the jaws.
2. We put the beveled part of the sponges close to the eye (positioning the shell vertically).
3. Through this gap we look at the vertically located thread of the burning lamp.
4. We observe iridescent stripes parallel to it on both sides of the thread.
5. We change the width of the slot in the range of 0.05 - 0.8 mm. When passing to narrower slits, the bands move apart, become wider, and form distinct spectra. When viewed through the widest slit, the fringes are very narrow and close to each other.
6. Pupils draw what they see in their notebooks.

Experimental work No. 5.

“Observation of light diffraction on kapron fabric”.

Equipment: a lamp with a straight filament, nylon fabric 100x100mm in size (Figure 10)

1. We look through the nylon fabric at the thread of a burning lamp.
2. We observe a “diffraction cross” (a pattern in the form of two diffraction bands crossed at a right angle).
3. Pupils draw in a notebook the picture they see (diffraction cross).

Explanation: A white diffraction peak is visible in the center of the crust. At k=0, the wave path difference is equal to zero, so the central maximum is white.

The cross is obtained because the threads of the fabric are two diffraction gratings folded together with mutually perpendicular slots. The appearance of spectral colors is explained by the fact that white light consists of waves of different lengths. The diffraction maximum of light for different wavelengths is obtained at different locations.

Experimental work No. 6.

“Observation of the diffraction of light on a gramophone record and a laser disk”.

Equipment: straight filament lamp, gramophone record (see figure 11)

The gramophone record is a good diffraction grating.

1. We position the record so that the grooves are parallel to the lamp filament and observe the diffraction in reflected light.
2. We observe bright diffraction spectra of several orders.

Explanation: The brightness of the diffraction spectra depends on the frequency of the grooves applied to the record and on the angle of incidence of the rays. (see figure 12)

Almost parallel rays incident from the lamp filament are reflected from adjacent bulges between the grooves at points A and B. The rays reflected at an angle equal to the angle of incidence form an image of the lamp filament in the form of a white line. Rays reflected at other angles have a certain path difference, as a result of which the waves are added.

Let us observe diffraction on a laser disk in a similar way. (see figure 13)

The surface of a CD is a spiral track with a step comparable to the wavelength of visible light. Diffraction and interference phenomena appear on the fine-grained surface. The highlights of CDs are iridescent.

Experimental work No. 7.

“Observation of diffraction coloration of insects from photographs”.

Equipment: (See drawings No. 14, 15, 16.)

Teacher: The diffraction coloration of birds, butterflies and beetles is very common in nature. A wide variety in shades of diffractive colors is characteristic of peacocks, pheasants, black storks, hummingbirds, and butterflies. The diffraction coloration of animals was studied not only by biologists but also by physicists.

Students look at photographs.

Explanation: The outer surface of the plumage of many birds and the upper body of butterflies and beetles are characterized by a regular repetition of structural elements with a period of one to several microns, forming a diffraction grating. For example, the structure of the central eyes of the peacock's tail can be seen in Figure No. 14. The color of the eyes changes depending on how the light falls on them, at what angle we look at them.

Control questions (each student receives a card with a task - answer the questions in writing ):

1. What is light?
2. Who proved that light is an electromagnetic wave?
3. What is the speed of light in vacuum?
4. Who discovered the interference of light?
5. What explains the iridescent coloration of thin interference films?
6. Can light waves from two incandescent bulbs interfere? Why?
7. Why is a thick layer of oil not iridescent?
8. Does the position of the main diffraction maxima depend on the number of grating slits?
9. Why does the apparent iridescent color of a soap film change all the time?

Homework (in groups, taking into account the individual characteristics of students).

– Prepare a report on the topic “Vavilov’s Paradox”.

– Compose crossword puzzles with the keywords “interference”, “diffraction”.

Literature:

1. Arabadzhi V.I. Diffraction coloration of insects / “Quantum” No. 2, 1975
2. Volkov V.A. Universal lesson developments in physics. Grade 11. - M.: VAKO, 2006.
3. Kozlov S.A. On some optical properties of CDs. / “Physics at school” No. 1, 2006
4. CDs / “Physics at School” No. 1, 2006
5. Myakishev G.Ya., Bukhovtsev B.B. Physics: Proc. for 11 cells. avg. school - M .: Education, 2000
6. Fabrikant V.A. Vavilov's paradox / "Quantum" No. 2, 1971
7. Physics: Proc. for 11 cells. avg. school / N.M. Shakhmaev, S.N. Shakhmaev, D.Sh. Shodiev. - M .: Education, 1991.
8. Physical Encyclopedic Dictionary / "Soviet Encyclopedia", 1983.
9. Frontal laboratory classes in physics in grades 7 - 11 of educational institutions: Book. for the teacher / V.A. Burov, Yu.I. Dik, B.S. Zworykin and others; Ed. V.A. Burova, G.G. Nikiforova. - M .: Education: Proc. lit., 1996

Lab #13

Subject: "Observation of interference and diffraction of light"

Goal of the work: experimentally study the phenomenon of interference and diffraction.

Equipment: an electric lamp with a straight filament (one per class), two glass plates, a glass tube, a glass with a soap solution, a wire ring with a handle with a diameter of 30 mm, a CD, a caliper, nylon fabric.

Theory:

Interference is a phenomenon characteristic of waves of any nature: mechanical, electromagnetic.

Wave interference – addition in space of two (or several) waves, in which at its different points an amplification or attenuation of the resulting wave is obtained.

Typically, interference is observed when the superposition of waves emitted by the same light source, which came to a given point in different ways. It is impossible to obtain an interference pattern from two independent sources, since molecules or atoms emit light in separate trains of waves, independently of each other. Atoms emit fragments of light waves (trains), in which the phases of oscillations are random. Tsugi are about 1 meter long. Wave trains of different atoms are superimposed on each other. The amplitude of the resulting oscillations changes chaotically with time so quickly that the eye does not have time to feel this change of pictures. Therefore, a person sees the space evenly lit. To form a stable interference pattern, coherent (matched) wave sources are needed.

coherent called waves that have the same frequency and a constant phase difference.

The amplitude of the resulting displacement at point C depends on the difference in the path of the waves at a distance d2 – d1.

Maximum condition

, (Δd=d 2 -d 1 )

Where k=0; ± 1; ±2; ± 3 ;…

(the difference in the path of the waves is equal to an even number of half-waves)

Waves from sources A and B will come to point C in the same phases and “amplify each other”.

φ A \u003d φ B - phases of oscillations

Δφ=0 - phase difference

A=2X max

Minimum condition

, (Δd=d 2 -d 1)

Where k=0; ± 1; ±2; ± 3;…

(the difference in the path of the waves is equal to an odd number of half-waves)

Waves from sources A and B will come to point C in antiphase and "extinguish each other".

φ A ≠φ B - oscillation phases

Δφ=π - phase difference

A=0 is the amplitude of the resulting wave.

interference pattern– regular alternation of areas of high and low light intensity.

Light interference- spatial redistribution of the energy of light radiation when two or more light waves are superimposed.

Due to diffraction, the light deviates from a rectilinear propagation (for example, near the edges of obstacles).

Diffraction – the phenomenon of wave deviation from rectilinear propagation when passing through small holes and rounding small obstacles by the wave.

Diffraction manifestation condition: d< λ , Where d- the size of the obstacle, λ - wavelength. The dimensions of the obstacles (holes) must be smaller than or commensurate with the wavelength.

The existence of this phenomenon (diffraction) limits the scope of the laws of geometric optics and is the reason for the limiting resolution of optical instruments.

Diffraction grating- an optical device, which is a periodic structure of a large number of regularly arranged elements on which light is diffracted. Strokes with a profile defined and constant for a given diffraction grating are repeated at regular intervals d(lattice period). The ability of a diffraction grating to decompose a beam of light incident on it into wavelengths is its main property. There are reflective and transparent diffraction gratings. In modern devices, mainly reflective diffraction gratings are used..

The condition for observing the diffraction maximum:

d sinφ=k λ, Where k=0; ± 1; ±2; ± 3; d- grating period , φ - the angle at which the maxima are observed, and λ - wavelength.

From the maximum condition it follows sinφ=(k λ)/d.

Let k=1, then sinφ cr =λ cr /d And sinφ f =λ f /d.

It is known that λ cr >λ f, hence sinφ cr>sinφ f. Because y= sinφ f - the function is increasing, then φ cr >φ f

Therefore, the violet color in the diffraction spectrum is located closer to the center.

In the phenomena of interference and diffraction of light, the law of conservation of energy is observed. In the area of ​​interference, light energy is only redistributed without being converted into other types of energy. The increase in energy at some points of the interference pattern relative to the total light energy is compensated by its decrease at other points (total light energy is the light energy of two light beams from independent sources). Light stripes correspond to energy maxima, dark stripes correspond to energy minima.

Progress:

Experience 1.Dip the wire ring in the soap solution. A soap film is formed on the wire ring.

Position it vertically. We observe light and dark horizontal stripes that change in width as the film thickness changes.

Explanation. The appearance of light and dark bands is explained by the interference of light waves reflected from the film surface. triangle d = 2h. The difference in the path of light waves is equal to twice the thickness of the film. When placed vertically, the film has a wedge-shaped shape. The difference in the path of light waves in its upper part will be less than in its lower part. In those places of the film where the path difference is equal to an even number of half-waves, bright stripes are observed. And with an odd number of half-waves - dark stripes. The horizontal arrangement of the stripes is explained by the horizontal arrangement of lines of equal film thickness.

We illuminate the soap film with white light (from the lamp). We observe the coloration of light bands in spectral colors: at the top - blue, at the bottom - red.

Explanation. This coloration is explained by the dependence of the position of the light bands on the wavelength of the incident color.

We also observe that the bands, expanding and retaining their shape, move down.

Explanation. This is due to a decrease in film thickness, as the soap solution flows down under the action of gravity.

Experience 2. Blow a soap bubble with a glass tube and examine it carefully. When illuminated with white light, observe the formation of colored interference rings, colored in spectral colors. The top edge of each light ring is blue, the bottom is red. As the film thickness decreases, the rings, also expanding, slowly move down. Their annular shape is explained by the annular shape of lines of equal thickness.

Answer the questions:

1. Why are soap bubbles iridescent?
2. What shape are the rainbow stripes?
3. Why does the color of the bubble change all the time?

Experience 3. Thoroughly wipe two glass plates, put together and squeeze with your fingers. Due to the non-ideal shape of the contacting surfaces, the thinnest air voids are formed between the plates.

When light is reflected from the surfaces of the plates that form the gap, bright iridescent stripes appear - ring-shaped or irregular in shape. When the force that compresses the plates changes, the arrangement and shape of the strips change. Draw the pictures you see.

Explanation: The surfaces of the plates cannot be perfectly even, so they touch only in a few places. Around these places, the thinnest air wedges of various shapes are formed, giving a picture of interference. In transmitted light, the maximum condition 2h=kl

Answer the questions:

1. Why are bright iridescent ring-shaped or irregularly shaped stripes observed at the points of contact of the plates?
2. Why does the shape and location of the interference fringes change with pressure?

Experience 4.Examine carefully from different angles the surface of the CD (which is being recorded).

Explanation: The brightness of the diffraction spectra depends on the frequency of the grooves deposited on the disk and on the angle of incidence of the rays. Almost parallel rays incident from the lamp filament are reflected from adjacent bulges between the grooves at points A and B. The rays reflected at an angle equal to the angle of incidence form an image of the lamp filament in the form of a white line. Rays reflected at other angles have a certain path difference, as a result of which the waves are added.

What are you observing? Explain the observed phenomena. Describe the interference pattern.

The surface of a CD is a spiral track with a pitch commensurate with the wavelength of visible light. On a fine-structured surface, diffraction and interference phenomena appear. The highlights of CDs are iridescent.

Experience 5. We shift the slider of the caliper until a gap of 0.5 mm wide forms between the jaws.

We put the beveled part of the sponges close to the eye (placing the gap vertically). Through this gap we look at the vertically located thread of the burning lamp. We observe rainbow stripes parallel to it on both sides of the thread. We change the width of the slot in the range of 0.05 - 0.8 mm. When passing to narrower slits, the bands move apart, become wider, and form distinct spectra. When viewed through the widest slit, the fringes are very narrow and close to each other. Draw the picture you see in your notebook. Explain observed phenomena.

Experience 6. Look through the nylon fabric at the filament of a burning lamp. By turning the fabric around the axis, achieve a clear diffraction pattern in the form of two diffraction bands crossed at right angles.

Explanation: A white diffraction peak is visible in the center of the crust. At k=0, the wave path difference is equal to zero, so the central maximum is white. The cross is obtained because the threads of the fabric are two diffraction gratings folded together with mutually perpendicular slots. The appearance of spectral colors is explained by the fact that white light consists of waves of different lengths. The diffraction maximum of light for different wavelengths is obtained at different locations.

Sketch the observed diffraction cross. Explain the observed phenomena.

Record the output. Indicate in which of your experiments the phenomenon of interference was observed, and in which diffraction.

Control questions:

1. What is light?
2. Who proved that light is an electromagnetic wave?
3. What is called interference of light? What are the maximum and minimum conditions for interference?
4. Can light waves from two incandescent bulbs interfere? Why?
5. What is the diffraction of light?
6. Does the position of the main diffraction maxima depend on the number of grating slits?

Subject: Observation of the phenomena of interference and diffraction of light.

Goal of the work: experimentally study the phenomenon of interference and diffraction.

Equipment:

• glasses with a solution of soap;
• wire ring with a handle;
• nylon fabric;
• CD;
• incandescent lamp;
• calipers;
• two glass plates;
• blade;
• tweezers;
• nylon fabric.

Theoretical part

Interference is a phenomenon characteristic of waves of any nature: mechanical, electromagnetic. Wave interference is the addition of two (or several) waves in space, in which at its different points an amplification or weakening of the resulting wave is obtained. To form a stable interference pattern, coherent (matched) wave sources are needed. Coherent waves are waves that have the same frequency and constant phase difference.

Maximum Conditions Δd = ±kλ, minimum conditions, Δd = ± (2k + 1)λ/2 where k =0; ± 1; ±2; ± 3;...(the difference in the path of the waves is equal to an even number of half-waves

An interference pattern is a regular alternation of areas of increased and decreased light intensity. Light interference is the spatial redistribution of the energy of light radiation when two or more light waves are superimposed. Consequently, in the phenomena of interference and diffraction of light, the law of conservation of energy is observed. In the area of ​​interference, light energy is only redistributed without being converted into other types of energy. The increase in energy at some points of the interference pattern relative to the total light energy is compensated by its decrease at other points (total light energy is the light energy of two light beams from independent sources).
Light stripes correspond to energy maxima, dark stripes correspond to energy minima.

Diffraction is the phenomenon of wave deviation from rectilinear propagation when passing through small holes and rounding small obstacles by the wave. Condition for the manifestation of diffraction: d< λ, Where d- the size of the obstacle, λ - wavelength. The dimensions of the obstacles (holes) must be smaller than or commensurate with the wavelength. The existence of this phenomenon (diffraction) limits the scope of the laws of geometric optics and is the reason for the limiting resolution of optical instruments. A diffraction grating is an optical device that is a periodic structure of a large number of regularly arranged elements on which light is diffracted. Strokes with a profile defined and constant for a given diffraction grating are repeated at regular intervals d(lattice period). The ability of a diffraction grating to decompose a beam of light incident on it into wavelengths is its main property. There are reflective and transparent diffraction gratings. In modern devices, mainly reflective diffraction gratings are used. Condition for observing the diffraction maximum: d sin(φ) = ± kλ

Instructions for work

1. Dip the wire frame in the soap solution. Observe and draw the interference pattern in the soap film. When the film is illuminated with white light (from a window or a lamp), light stripes are colored: at the top - blue, at the bottom - red. Use a glass tube to blow a soap bubble. Watch him. When illuminated with white light, the formation of colored interference rings is observed. As the film thickness decreases, the rings expand and move down.

Answer the questions:

1. Why are soap bubbles iridescent?
2. What shape are the rainbow stripes?
3. Why does the color of the bubble change all the time?

2. Thoroughly wipe the glass plates, put them together and squeeze with your fingers. Due to the non-ideal shape of the contacting surfaces, the thinnest air voids are formed between the plates, giving bright iridescent annular or closed irregularly shaped stripes. When the force compressing the plates changes, the location and shape of the bands change both in reflected and transmitted light. Draw the pictures you see.

Answer the questions:

1. Why are bright iridescent annular or irregularly shaped stripes observed in separate places of contact between the plates?
2. Why does the shape and location of the obtained interference fringes change with a change in pressure?

3. Lay a CD horizontally at eye level. What are you observing? Explain the observed phenomena. Describe the interference pattern.

4. Look through the nylon fabric at the filament of a burning lamp. By turning the fabric around the axis, achieve a clear diffraction pattern in the form of two diffraction bands crossed at right angles. Sketch the observed diffraction cross.

5. Observe two diffraction patterns when examining the filament of a burning lamp through a slit formed by the jaws of a caliper (with a slit width of 0.05 mm and 0.8 mm). Describe the change in the nature of the interference pattern when the caliper is smoothly rotated around the vertical axis (with a slit width of 0.8 mm). Repeat this experiment with two blades, pressing them against each other. Describe the nature of the interference pattern

Record your findings. Indicate in which of your experiments the phenomenon of interference was observed? diffraction?