Theme: Optics
Lesson: Practical work on the topic "Observation of interference and diffraction of light"
Name:"Observation of interference and diffraction of light".
Target: experimentally study the interference and diffraction of light.
Equipment: lamp with straight filament, 2 glass plates, wire frame, soap solution, caliper, thick paper, piece of cambric, nylon thread, clip.
Experience 1
Observation of the interference pattern using glass plates.
We take two glass plates, before that we carefully wipe them, then fold them tightly and squeeze. That interference pattern, which we see in the plates, needs to be sketched.
To see the change in the picture from the degree of compression of the glasses, it is necessary to take the clamping device and compress the plates with the help of screws. As a result, the interference pattern changes.
Experience 2
Interference on thin films.
To observe this experiment, let's take soapy water and a wire frame, then see how a thin film is formed. If the frame is lowered into soapy water, then after lifting it, a soap film is visible in it. By observing this film in reflected light, interference fringes can be seen.
Experience 3
Soap bubble interference.
For observation, we use a soapy solution. We blow soap bubbles. The way the bubbles shimmer is the interference of light (see Fig. 1).
Rice. 1. Light interference in bubbles
The picture that we observe may look like this (see Fig. 2).
Rice. 2. Interference pattern
This is white light interference when we put a lens on glass and illuminate it with plain white light.
If you use light filters and illuminate with monochromatic light, then the interference pattern changes (the alternation of dark and light stripes) (see Fig. 3).
Rice. 3. Using filters
We now turn to the observation of diffraction.
Diffraction is a wave phenomenon inherent in all waves, which is observed at the edge parts of any objects.
Experience 4
Diffraction of light by a small narrow slit.
Let's create a gap between the jaws of the caliper by moving its parts with the help of screws. In order to observe the diffraction of light, we clamp a sheet of paper between the lips of the caliper so that this sheet of paper can then be pulled out. After that, we bring this narrow slit perpendicularly close to the eye. When observing a bright light source (an incandescent lamp) through the slit, one can see the diffraction of light (see Fig. 4).
Rice. 4. Diffraction of light by a thin slit
Experience 5
Diffraction on thick paper
If you take a thick sheet of paper and make an incision with a razor, then by bringing this cut of paper close to the eye and changing the location of the adjacent two leaves, you can observe the diffraction of light.
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, light diffraction is visible (see Fig. 5).
Change diffraction patterns s depends on the size of the hole.
Rice. 5. Diffraction of light by a small hole
Experience 7
Diffraction of light on a piece of dense transparent fabric (nylon, cambric).
Let's take a cambric ribbon and, placing it at a small distance from the eyes, look through the ribbon at a bright light source. We will see diffraction, i.e. multi-colored stripes and a bright cross, which will consist of lines of the diffraction spectrum.
The figure shows photographs of the diffraction that we observe (see Fig. 6).
Rice. 6. Diffraction of light
Report: it should present the patterns of interference and diffraction that were observed during the work.
The change in lines characterizes how one or another procedure of refraction and addition (subtraction) of waves occurs.
Based on the diffraction pattern obtained from the slit, a special device was created - diffraction grating. It is a set of slits through which light passes. This device is needed in order to conduct detailed studies of light. For example, using a diffraction grating, you can determine the wavelength of light.
- Physics().
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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 out 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?
APPLICATION
Diffraction of light is the deviation of light rays from a straight path when passing through narrow gaps, small openings or when avoiding 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 of white light, a rainbow stripe.
Diffraction grating is an optical instrument for measuring the wavelength of light.
The diffraction grating is a set a large number very narrow slits 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 the interference pattern that occurs when light is reflected 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.
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 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 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 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, hence 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 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 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 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.
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 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 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 of 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.
Control 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?
Goal of the work: Investigate the dependence of the oscillation period of a thread pendulum on the length of the thread and of a spring pendulum on the mass of the load.
Equipment: tripod, ruler, thread weight, set of weights, stopwatch, spring.
Explanations for work
A filament pendulum consists of a weight m suspended on a weightless inextensible thread of length l. The dependence of the period of oscillation of the thread pendulum on the length of the thread is expressed
formula: .
A spring pendulum consists of a load of mass m suspended by a spring of rigidity k. The dependence of the oscillation period of a spring pendulum on the mass of the load is expressed by the formula: .
Tasks
Assemble the string pendulum.
Draw a diagram of the experience.
Measure the time of 15-20 oscillations (the number of oscillations in all experiments should be the same).
Changing the length of the thread (only increasing or only decreasing), measure the oscillation time 4 more times.
Fill in Table 1 of measurement and calculation results:
| experience number | Thread length, l, m | Time interval, t, s | Oscillation period, | Oscillation period, |
| 1 | ||||
| 2 | ||||
| 3 | ||||
| 4 | ||||
| 5 |
7. Check the calculations using the formula: .
Plot a graph of the period of oscillation of a thread pendulum as a function of the length of the thread.
Assemble the spring pendulum.
Draw a diagram of the experience.
Determine the stiffness of the spring:
b) determine the force F , and also measure the corresponding extension of the spring X;
c) calculate the spring stiffness coefficient using the formula: .
Measure the time of 10-15 oscillations (the number of oscillations in all experiments should be the same).
Changing the mass of the load (only increasing or only decreasing), measure the oscillation time 4 more times.
Determine the period of oscillation of the pendulum in each experiment using the formula: T=t/N.
Fill in Table 2 of measurement and calculation results:
| experience number | Weight of cargo, m, kg | Time interval, t, s | Oscillation period, | Oscillation period, |
| 1 | ||||
| 2 | ||||
| 3 | ||||
| 4 | ||||
| 5 |
Control questions
What vibrations are called free?
How is the period of oscillation related to frequency?
Will the period of oscillation of a body change if it is placed from air into water?
What is the physical meaning of the oscillation phase?
Literature
The job is 2 hours
Lab #11
Subject: Phenomena Researchinterference and diffraction of light
Goal of the work: explore characteristics interference and diffraction of light.
Equipment: matches, a spirit lamp, a ball of cotton wool on a wire in a test tube moistened with a solution of sodium chloride, a wire ring with a handle, a glass with a solution of soap, a glass tube, glass plates -2 pcs., CD-ROM, caliper, lamp with a straight filament, nylon fabric black color.
Explanations for work
Observation of light interference
To observe interference with monochromatic radiation, a lump of cotton wool moistened with a solution of sodium chloride is introduced into the flame of an alcohol lamp. In this case, the flame is colored in yellow. By dipping a wire ring into a soap solution, a soap film is obtained, placed vertically and viewed against a dark background under the yellow light of a spirit lamp. Observe the formation of dark and yellow horizontal stripes and a change in their width as the film thickness decreases.
In those places of the film where the path difference of the coherent rays is equal to an even number of half-waves, light bands are observed, and at an odd number of half-waves, dark bands are observed.
When the film is illuminated with white light (from a window or a lamp), light stripes are colored: blue at the top, red at the bottom. Using a glass tube, a small soap bubble is blown onto the surface of the soap solution. When illuminated with white light, the formation of colored interference rings is observed. As the film thickness decreases, the rings expand and move down.
Interference is also observed when considering the contact surface of two glass plates compressed with each other.
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.
The phenomenon of interference of reflected light rays is especially clearly observed when considering the surface of a CD.
Light Diffraction Observation
The diffraction of light is manifested in the violation of the straightness of the propagation of light rays, the rounding of obstacles by waves, and the penetration of light into the region of a geometric shadow.
As an inhomogeneity of the medium, a gap between the jaws of the caliper is used in the work. Through this slit they look at the vertically located thread of the burning lamp. At the same time, rainbow stripes are visible on both sides of the thread, parallel to it. As the slit width decreases, the bands move apart, become wider, and form clearly distinguishable spectra. This effect is observed especially well with a smooth rotation of the caliper around the vertical axis.
Another diffraction pattern is observed on a thin filament. A frame with a thread is placed on the background of a burning lamp parallel to the filament. By removing and bringing the frame closer to the eye, a diffraction pattern is obtained, when light and dark stripes are located on the sides of the thread, and in the middle, in the region of its geometric shadow, a light strip is observed.
A diffraction pattern can be observed on kapron fabric. In kapron fabric there are two distinguished mutually perpendicular directions. Turning the fabric around the axis, look through the fabric at the filament of a burning lamp, achieving a clear diffraction pattern in the form of two diffraction bands crossed at right angles (diffraction cross). A diffraction maximum of white color is visible in the center of the cross, and several colors in each band.
Tasks
Explore for yourself guidelines for doing laboratory work.
Perform experiments on observing the interference of light.
Experience 2: examine the soap film when illuminated by white light (from a window or lamp).
Explain the order of alternation of colors in the interference pattern when the film is illuminated with white light.
Experience 3: 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.
Experience 4: squeeze two glass plates together. Describe the observed interference pattern. Determine the change in the interference pattern with increasing force compressing the plates.
Experience 5: take a CD and direct the light beams from the light bulb onto it. Describe the interference pattern when a CD is illuminated.
Perform experiments on observing the diffraction of light.
Experience 2: take a nylon fabric and look through it 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.
Prepare a report, it should contain: the name of the topic and purpose of the work, drawings of interference and diffraction patterns, their description and explanations, conclusions on the work.
Control questions
Define interference and diffraction of light.
Under what conditions is the interference pattern observed?
Name the condition of coherence of light waves.
That proves the phenomenon of light interference.
Give examples of light diffraction.
Literature
Dmitrieva V. F. Physics for professions and specialties of a technical profile: a textbook for educational institutions early and avg. prof. education. - M.: Publishing Center "Academy", 2014;
Samoilenko P.I. Physics for professions and specialties of the socio-economic profile: a textbook for educational institutions of primary and secondary prof. education. - M.: Publishing Center "Academy", 2013;
Kasyanov V.D. Notebook for laboratory work. Grade 10. - M .: Bustard, 2014.
The job is 2 hours
Lab #12
Subject: Measuring the wavelength of light with a diffraction grating
Goal of the work: measure the wavelength of light using a diffraction grating.
Equipment: light source, diffraction grating, device for measuring the wavelength of light.
Explanations for work
A diffraction grating is used to decompose light into a spectrum and measure the wavelength of light. The simplest diffraction grating is a glass plate on which, with the help of an accurate dividing machine, scratches are applied parallel to each other and narrow intact strips are installed. The scratched places are opaque to light, and light waves approaching the grating go around these scratches. It is customary to call the grating period d the sum of the sizes of the transparent and opaque bands. For example, if the diffraction grating has 100 lines per 1 mm, then the grating period is d=0.01 mm.
Let a parallel monochromatic (all waves have the same wavelength) beam of light fall on the grating along the normal. Light passing through narrow slits experiences diffraction, and the rays deviate from the original direction at different angles. Each slit of the diffraction grating can be considered an independent source of coherent radiation. Therefore, at each point of the screen, the summation of numerous rays coming from each slit of the diffraction grating will occur, and their interference will occur. Since the initial light wave falls on the grating normally, the initial phases of all beams are the same. The distance from the grating to the screen is much greater than its dimensions, so the rays from different slits, infragrating at the same angle Θ, will hit the same point on the screen. Its coordinate b is determined by the expression:
sin Θ ≈ tg Θ = b/ a,
where it is assumed that the diffraction angles are small, so you can replace the value of the sine with the tangent. The path difference of these rays is related to the diffraction angle by the relation:
If the difference in the path of the rays is equal to an integer number m=1,2,3, ... wavelengths, then: ∆ = m λ, and when added, they mutually reinforce each other, and a maximum is observed, which is called the main diffraction maximum of order m.
The diffraction angles corresponding to the main maxima are determined from the formula:
∆=d sin Θm = mλ.
The right side of this equation is called the diffraction grating equation. As can be seen from it, the position of the diffraction maximum depends on the wavelength and the order of the maximum m: the greater the wavelength of light and the order number, the greater the diffraction angle. Therefore, when the grating is illuminated with white light, the rays with various lengths waves are diffracted different angles, and as a result, the diffraction maximum is converted into a spectrum. In this case, a set of spectra is formed on the screen, which can partially overlap each other (one spectrum corresponds to each value of the diffraction order m). The limiting number of spectra that can be obtained using a grating determines the ratio: m max =d/λ.
When m=0, the image is created by a beam parallel to the incident light beam (Θ=0), and the actions of all beams are summed up, regardless of wavelength, so a white light stripe is observed in the center.
Tasks
Study the guidelines for performing laboratory work on your own.
Place the diffraction grating in the frame of the instrument.
Looking through the diffraction grating, point the instrument at the light source so that the latter is visible through the narrow aiming slit of the screen. In this case, diffraction spectra of several orders of magnitude appear on both sides of the slit against a black background. If the spectra are tilted, rotate the grating by some angle until the skew is eliminated. Determine the position of the red and violet borders of the spectrum for the 1st and 2nd orders. Measure the distance from the shield to the grating and calculate the wavelength for violet and red light using the formula:
Distance A is from the grating to the screen, the distance b is from the slit to the spectrum line of the wave being determined, m is the order of the spectrum, d is the grating constant.
Repeat measurements of violet and red light lengths at a shorter distance A from the diffraction grating.
Record the results of measurements and calculations in the table:
| experience number | lattice constant, d, mm | Spectrum order, m | Distance from grating to scale, A, mm | Deviation value, b, mm | Light wave length, λ, mm |
||
| violet | red | violet | red |
||||
| 1 | |||||||
| 2 | |||||||
Prepare a report, it should contain: the name of the topic and the purpose of the work, a list necessary equipment, calculated ratios, a table with the results of measurements and calculations, a conclusion on the work.
Orally answer the control questions.
Control questions
Which wavelengths (red or violet light) diffract more and why?
Define the diffraction of light.
How does the diffraction angle depend on the grating period?
What is a diffraction grating used for?
How to determine the wavelength of light?
Literature
Dmitrieva VF Physics for professions and specialties of a technical profile: a textbook for educational institutions beginning. and avg. prof. education. - M.: Publishing Center "Academy", 2014;
Samoilenko P.I. Physics for professions and specialties of the socio-economic profile: a textbook for educational institutions of primary and secondary prof. education. - M.: Publishing Center "Academy", 2013;
Kasyanov VD Notebook for laboratory work. Grade 10. - M .: Bustard, 2014.
The job is 2 hours
