Laboratory work № 13
Topic: "Observation of interference and diffraction of light"
Objective: 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 superimposing waves emitted by the same light source that come to a given point 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 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. AT modern appliances 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, Consequently sinφ cr>sinφ f. Because y= sinφ f - the function is increasing, then φ cr >φ f
That's why purple 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 coloration light stripes into spectral colors: above - blue, below - 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 has Blue colour, the lower one 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 bubble are 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.
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 compressing 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. The thinnest air wedges form around these places. various shapes giving a picture of the 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.Look carefully under different angles the surface of the CD (which is being written to).
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 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 one another. Draw the picture you see in your notebook. Explain observed phenomena.
Experience 6. Look through nylon fabric on 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 diffraction maximum is visible at the center of the crust white color. 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 various 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.
Test questions:
- What is light?
- Who proved that light is an electromagnetic wave?
- What is called interference of light? What are the maximum and minimum conditions for interference?
- Can light waves from two incandescent bulbs interfere? Why?
- What is the diffraction of light?
- Does the position of the main diffraction maxima depend on the number of grating slits?
Task 1. Observation of light interference on an air film.
1. Wipe the glass plates thoroughly, put them together and squeeze with your fingers.
2. Examine the plates in reflected light against a dark background (they must be positioned so that too bright reflections from windows or white walls do not form on the glass surface).
3. In some places where the plates come into contact, bright iridescent ring-shaped or irregularly shaped stripes can be observed.
4. Notice changes in the shape and location of the obtained interference fringes with a change in pressure.
5. Try to see the interference pattern in transmitted light.
6. Draw the pictures you see.
Answer the questions:
a) Why are bright iridescent annular or irregularly shaped stripes observed in separate places of contact between the plates?
b) Why does the shape and location of the obtained interference fringes change with a change in pressure?
Task 2. Observation of light interference on a soap film.
1. Make a soapy solution.
2. Get a soap film on the wire ring and place it vertically.
3. In a darkened classroom, observe the appearance of light and dark bands on the film.
4. Illuminate the soap film with light from a lamp or flashlight.
5. Observe the coloration of light bands in spectral colors.
6. Count the number of bands of the same color that are simultaneously observed on the film.
7. Determine if the orientation and shape of the stripes change when the frame is rotated in the vertical plane.
8. Draw the pictures you see.
Answer the questions:
a) What explains the presence of light and dark bands at the beginning of the experiment?
b) Why do spectral colors appear when the film is illuminated with light?
c) Why do the stripes, expanding and retaining their shape, flow down?
Task 3. Observation of light interference on a soap bubble.
1. Blow a soap bubble.
2. When illuminated with white light, observe the formation of colored interference rings.
Answer the questions:
a) Why are soap bubbles iridescent?
b) Why does the color of the bubble change all the time?
c) What shape are the rainbow stripes?
Task 4. Temper colors.
1. Take a safety razor blade with tweezers and heat it over a burner flame.
2. Sketch the observed picture.
Answer the questions:
a) What phenomenon did you observe?
b) How can it be explained?
c) What colors and in what order appeared on the blade when it was heated?
Study of the diffraction of light.
Task 1. Observation of light diffraction by a narrow slit.
1. Install a gap 0.5 mm wide between the jaws of the caliper.
2. Attach the slit close to the eye, placing it vertically.
3. Looking through the slit at a vertically located luminous lamp filament, observe rainbow stripes (diffraction spectra) on both sides of the filament.
4. By changing the slit width from 0.5 to 0.8 mm, notice how this change affects the diffraction spectra.
5. Draw the picture you see in your notebook.
Task 2. Observation of diffraction on nylon fabric.
1. Look through the nylon fabric at the filament of a burning lamp.
2. By turning the fabric around the axis, achieve a clear diffraction pattern in the form of two diffraction bands crossed at right angles.
3. Sketch the observed diffraction cross.
Answer the questions:
a) Why did this diffraction cross come about?
b) What explains the appearance of spectral colors?
Task 3. Observation of light diffraction on a laser disk.
1. Lay a CD horizontally at eye level.
2. Draw this picture.
Answer the questions:
a) What is on the disk?
b) What phenomena did you observe?
APPENDIX
Diffraction of light- this is the deviation of light rays from rectilinear propagation when passing through narrow slots, small openings or when bending around small obstacles.
The phenomenon of light diffraction proves that light has wave properties. To observe diffraction, you can:
Pass light from a source through a very small hole, or place the screen at a great distance from the hole. Then a complex picture of light and dark concentric rings is observed on the screen.
Or direct light onto a thin wire, then light and dark stripes will be observed on the screen, and in the case white light- rainbow stripe
Diffraction grating is an optical instrument for measuring the wavelength of light.
A diffraction grating is a collection of a large number of very narrow gaps separated by opaque gaps.
If a monochromatic wave falls on the grating, then the slits (secondary sources) create coherent waves. A converging lens is placed behind the grille, then a screen. As a result of the interference of light from different grating slits, a system of maxima and minima is observed on the screen.
The path difference between the waves from the edges of adjacent slots is equal to the length of segment AC. If an integer number of wavelengths fits on this segment, then the waves from all slots will amplify each other. When using white light, all maxima (except the central one) have a rainbow color.
d = a + b - grating period
a - slot width; b - length
d = 1/N is the diffraction grating constant.
N - Number of strokes.
φ - angle of deflection of light waves due to diffraction
φ = kλ - diffraction grating formula.
k - Order of maximum (0, ±1, ±2, ...)
λ = - wavelength
Light interference- spatial redistribution of the light flux when two (or several) coherent light waves are superimposed, as a result of which intensity maxima appear in some places, and minima in others (interference pattern).
Maximum condition: Minimum condition:
Application of light interference:
1. Length measurement with very high accuracy; this made it possible to give an easily reproducible and fairly accurate definition of the unit of length - the meter, depending on the wavelength of the orange krypton line. Interference comparators allow you to compare sizes up to 1 meter with an accuracy of 0.05 microns; smaller dimensions can be measured with even greater accuracy. Such a high accuracy is due to the fact that a change in the path difference by a tenth of a wavelength noticeably shifts the interference fringes.
2. The effect of a large number of optical devices under common name interferometers, which are used for various measurements. In the optomechanical industry, interferometers are used to control the quality of optical systems and to control the surface of individual optical parts. In the metalworking industry - to control the cleanliness of the processing of metal surfaces. The study and control of the polishing of mirror surfaces is carried out with an accuracy of hundredths of a wavelength.
3. Using the phenomenon of interference, a number of the most important quantities characterizing substances are determined: the expansion coefficient solids(dilatometers), refractive index of gaseous, liquid and solid bodies (refractometers), etc. Interference dilatometers allow fixing the elongation of the sample by 0.02 µm.
4. Interference spectroscopes are widely used to study the spectral composition of the radiation of various substances.
5. Through the interference of polarized beams, the values of internal stresses in various parts are determined (photoelasticity method).
The first experiment to observe the interference of light in the laboratory belongs to I. Newton. He observed an interference pattern arising from the reflection of light in a thin air gap between a flat glass plate and a plano-convex lens with a large radius of curvature. The interference pattern looked like concentric rings, called Newton's rings.
Temper colors.
Temper colors - iridescent color that appears on the clean surface of heated steel as a result of the formation of a thin oxide film on it. The thickness of the film depends on the heating temperature of the steel: films of different thicknesses reflect light rays in different ways, which is the reason for certain tint colors (see table). On alloyed (into which other metals are introduced to impart certain properties) steels, the same tint colors appear at higher temperatures.
1. The purpose of the work: to study characteristics interference and diffraction of light.
2. Literature:
2.1. Kasyanov V.A. Physics. Grade 11: textbook for general education educational institutions. - M., 2003. Paragraphs 44, 45, 47.
2.2. Abstract of lectures on the subject "Physics".
3. Preparation for work:
3.1. Answer self-test questions to obtain a work permit:
3.1.1. What phenomenon is called interference?
3.1.2. What waves are called coherent? Name the methods for obtaining coherent wave sources.
3.1.3. What phenomenon is called diffraction?
3.1.4. What is the Huygens-Fresnel principle?
3.2. Prepare a report form in accordance with paragraph 6.
4. List necessary equipment:
4.2. Electronic edition"Laboratory work in physics grades 10-11": Bustard, 2005. Laboratory work No. 12.
5. Order of performance of work:
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Turn on the PC. Set laboratory work No. 12. Consider the equipment for the experiment (Fig. 1).
5.2. Light the spirit lamp (2). Bring into the flame a piece of cotton wool (3) moistened with a solution of sodium chloride.
5.3. Dip the wire ring into the soap solution to form a soapy film.
5.4. Sketch the interference pattern obtained on the film when illuminated with the yellow light of a spirit lamp (Fig. 2). Explain the sequence colors in the interference pattern when the film is illuminated with white light.
5.5. Use a glass tube to blow a small soap bubble on the surface of the soap solution. Explain the reason for the downward movement of the interference rings.
5.6. Describe the interference pattern observed from two compressed glass plates. How does the observed pattern change as the force that presses the plates together increases?
5.7. 
Describe the interference pattern when a CD is illuminated. Sketch two diffraction patterns observed when examining the filament of a burning lamp through the slit of a caliper (with a slit width of 0.05 and 0.8 mm). Describe the change in the nature of the interference pattern with a smooth rotation of the caliper around the vertical axis with a slit width of 0.8 mm. Place the frame with the thread against the background of a burning lamp parallel to the filament (Fig. 3). Moving the frame relative to the eye, ensure that in the middle, in the area of the geometric shadow of the thread, a light strip is observed. Sketch the diffraction pattern observed for a thin filament.
5.8. Look through the black 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, describe it.
6.1. Number and title of work.
6.2. Objective.
6.3. Figure of the interference pattern (Fig. 2) and its explanation.
6.4. Explanation of the interference pattern on the surface of a soap bubble.
6.5. Drawing of an interference pattern observed from two compressed glass plates. Explanation of its change during compression of the plates.
6.6. Description of the interference pattern when illuminating a CD.
6.7. Drawing of two diffraction patterns on a slit of 0.05 and 0.8 mm. Describe its change with a smooth rotation of the slot around the vertical axis.
6.8. Drawing a diffraction pattern on a thin thread.
6.9. Drawing of a diffraction pattern on a kapron thread. diffraction cross.
Laboratory work number 13.
Topic: Observation of the phenomena of interference and diffraction of light.
Objective: experimentally study the phenomenon of interference and diffraction.
Equipment:
- glasses with a solution of soap;
- wire ring with a handle;
- nylon fabric;
- compact disc;
- 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:
- Why are soap bubbles iridescent?
- What shape are the rainbow stripes?
- 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:
- Why are bright iridescent annular or irregularly shaped stripes observed in separate places of contact between the plates?
- 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?
