Teacher Notes

Laser Pointer Education

Student Laboratory Kit

Materials Included In Kit

Cap, black, with holes
Color filters, red, green and blue
Colored paper, red, green and blue
Diffraction grating, holographic
Dish, semicircular
Lens, cylindrical, 1" long, ¼" diameter
Mirror, flat
Mirror stand
Polarizing filter, 5 cm x 5 cm
Protractor Master Sheet

Additional Materials Required

Water, 250-mL*
Water (Activities 4, 5 and 6)†
Aquarium tank (Activity 6)†
Balloon, or latex sheeting (rubber dam) (Activity 8)†
Beaker, 250-mL*
Bottle, 2-Liter (Activity 5)†
Can opener (Activity 8)†
Chalkboard eraser dust, or coffee creamer (Activities 1 and 7)†
Coffee can (Activity 8)†
Folder, manila*
Ice cube (Activity 2)†
Incandescent lightbulb, frosted (Activity 3)†
Laser pointer, pen-type*
Lumirod (Activities 4 and 5)†
Pen, permanent, black*
Razor blade or diffraction grating (Activity 7)†
Ruler*
Sugar, 1-lb bag (Activity 6)†
Tape, transparent*
Test tube, 150-mm, or round-bottom flask, 1-L (Activity 4)†
The Mirage (Catalog No. AP4739) (Activity 9)†
Thumb tack or nail (Activity 5)†
*for each lab group
for optional Teacher Demonstrations

Prelab Preparation

Optional Teacher Demonstrations

Part 1. Light Interaction

Activity 1. Light scattering

  1. Obtain some old chalkboard erasers and some chalk dust.
  2. Bang the erasers together to scatter the chalk dust into the air.
  3. Shine the laser beam through the chalk dust. Observe the visible laser beam in the dust cloud. (The result of light scattering.)
Activity 2. View the imperfections in an ice cube
  1. Shine the laser beam onto an ice cube.
  2. Observe the scattering pattern of the laser light near the edges and cracks in the ice cube.
Activity 3. Observing a lightbulb’s filament through scattering
  1. Shine the laser beam at a frosted incandescent lightbulb.
  2. Observe the scattered light making the lightbulb appear to glow red.
  3. The shadow of the filament will be projected on the dark side of the lightbulb.
Part 2. Reflection

Activity 4. Observing total internal reflection I
  1. Obtain a test tube or 1-L round-bottom flask and fill it three-quarters full with water.
  2. Use a Beral-type pipet to add a drop of milk to the water to make it slightly cloudy.
  3. Shine the laser beam through the bottom of the test tube, or a round-bottom flask.
  4. Adjust the angle of the beam to produce an “internal” reflection of the laser beam. Is there an angle that will produce more than one internal reflection—like in a fiber optic?
Part 3. Refraction

Activity 5. Observing total internal reflection II

  1. Obtain a 2-liter soda bottle and use a hot nail or thumb tack to poke a hole near the bottom of the bottle (see Figure 8).
    {12766_Preparation_Figure_8}
  2. Place a piece of tape over the hole and fill the bottle with water. The tape should prevent the water from leaking out of the bottle.
  3. Place the bottle near a sink and point the hole at the sink.
  4. Remove the tape from the bottle and allow the water to drain into the sink.
  5. Shine the laser beam directly at the hole from the side of the bottle opposite the hole (see Figure 8).
  6. Maintain the laser beam position directly on the hole and observe the water “light pipe.”
Activity 6. Refractive density aquarium (curving the laser beam)
  1. Obtain an empty aquarium and cover the bottom with enough sugar so that it is about 1 cm high.
  2. Slowly add water to the aquarium so as not to disturb the sugar layer significantly.
  3. Continue to add water until the aquarium is ½ to ¾ full.
  4. Allow the aquarium to sit undisturbed for a week to allow the sugar to slowly dissolve in the water and develop a concentration (density) gradient.
  5. After a week, test the density distribution by shining the laser into the water at a slight angle. The laser beam should bend towards the floor as the light beam is refracted into denser and denser sugar–water solution (see Figure 9).
    {12766_Preparation_Figure_9}
  6. Can you get the light beam to “bounce” across the bottom of the aquarium tank via internal reflections at the bottom of the tank and the graded refraction through the density gradient?
Part 4. Diffraction

Activity 7. Observing the diffracted beam

  1. Obtain a razor blade, support stand, buret clamp, and chalk dust or coffee creamer.
  2. Mount the razor blade in the buret clamp as shown in Figure 10.
    {12766_Preparation_Figure_10}
  3. Create a nice cloud of dust a few meters beyond the razor blade.
  4. Point the laser directly at the horizontal razor blade edge parallel to the ground and observe the diffracted light beams projecting vertically in the dust. The bright and dark bands will be visible in the cloud. (The reason the razor blade diffracts the laser beam is explained by Babinet’s principle: Coherent light shining on a thin slit or thin object of the same thickness will produce the same diffraction pattern.)

Activity 8. Visualizing sound waves

  1. Obtain an empty coffee can and a round balloon or latex sheeting (“rubber dam”).
  2. Use a can opener to cut out both ends of the coffee can.
  3. Cut the balloon in the middle so that it can be stretched over an opening of the coffee can.
  4. Stretch the balloon over one opening of the coffee can and secure it with tape or a rubber band.
  5. Obtain the flat mirror.
  6. Glue or tape the mirror to the center of the stretched balloon.
  7. Observe the mirror’s movements when you speak into the other side of the coffee can.
  8. Place the can next to a stereo speaker so that the open end is adjacent to the speaker (see Figure 11).
    {12766_Preparation_Figure_11}
  9. Turn on the radio and shine the laser beam onto the mirror.
  10. Observe the erratic motion of the reflected laser beam on the ceiling. The flexible balloon vibrates as a result of the sound (air pressure vibrations).

Activity 9. Shine a light on a “hologram”

  1. Obtain The Mirage (Flinn Catalog No. AP4739) and an object to place inside The Mirage.
  2. Point the laser at the projected image of the object above the hole in The Mirage. Notice how the laser beam appears to “hit” the image. (The curved mirrors reflect the laser beam onto the real object inside The Mirage, and the image of the object, with the laser beam dot, is projected above the hole in the dome.)

Safety Precautions

Do not aim the laser pointer directly into anyone’s eyes. The low power, coherent light can cause damage to the sensitive retina and may lead to permanent blindness. Make sure no stray laser light extends beyond the classroom to prevent any unintentional eye contact. When reflecting the laser light between mirrors, it is best to do this low to the floor to keep the reflected laser light below “normal” eye level. For people with sensitive eyes it is recommended that dark, IR-protective, safety glasses be worn. Care should be taken when handling razor blades. Chalk dust, and other finely divided powders are potentially flammable. Keep open flames away when using chalk dust. Follow all other normal laboratory safety guidelines.

Disposal

The materials may be saved for future use.

Teacher Tips

  • Enough materials are provided in this kit for one group of students. Additional kits may be purchased separately. These activities and/or experiments do not need to be completed all at once. The experiments and activities should be performed in accordance with the topics and pace of your classroom curriculum.
  • For best results, perform the experiments and activities in a darkened room.
  • Photocopy the Protractor Master Sheet before performing the experiments.
  • If it is difficult to see the laser beam in the water, add a drop of milk to the water to make the water slightly cloudy.
  • When the laser beam enters the semicircular dish at the midpoint of the flat side (the center of the complete circle), the refracted ray will strike the curved end of the dish “normal” to the surface (see Figure 12). Therefore, there will be no refraction of light when the light beam exits the water at the curved surface. The angle of the light beam traveling in the water will be the same as the light beam that exits the dish through the curved side and shines on the protractor sheet. This is why it is important to aim the laser beam at the center of the flat side of the semicircular dish.
    {12766_Tips_Figure_12}
  • The laser beam wavelength may vary from trial to trial because the wavelength depends on the condition of the transistor. Heat affects the transistor properties, and therefore the wavelength of the light. A laser that has been used continuously for several minutes may produce a light with a wavelength that is slightly different compared to when it was just turned on. Normal operating wavelength for a red pen-type laser is 645 ±30 nm.

Further Extensions

  • Extension for Experiment 6: An index of refraction plate (Flinn Catalog No. AP9329) can also be used in place of the semicircular dish filled with water. Also, different fluids such as mineral oil or corn syrup can be used with the semi-circular dish to determine the index of refraction of materials other than water. Avoid organic solvents such as acetone or hexane. These may dissolve or distort the plastic dish.
  • Use diffraction to explain and demonstrate Heisenburg’s Uncertainty Principle. Heisenburg’s Uncertainly Principle states that one can not know both the location and the velocity of a subatomic particle at the same time. When the light is directed through a vertical thin slit, the horizontal location of the photons is known at the slit opening. Since the horizontal position is known, the horizontal velocity exiting the slit will be random. Velocity is both speed and direction, and since the speed of light is constant, the horizontal direction of the light beam is unknown. Therefore, the laser light spreads out horizontally in all directions as it exits a thin slit, but not vertically. The bright and dark spots that are created are the result of the diffraction pattern of coherent (same wavelength) light passing through a thin slit. Small slits can be made by taping the sharp edge of two razor blades together, or by cutting a thin hole in a strip of black paper.
  • Extension for Teacher Demonstration Activities 4 and 5: A Lumirod (Flinn Catalog No. AP6325) can be used as a light pipe instead of a water-filled test tube or a water light pipe.
  • Holographic diffraction gratings are manufactured to have a sinusoidal groove profile making them very efficient compared to standard ruled diffraction gratings. Because of the sinusoidal groove, only the zero and first order diffraction pattern can be observed. The higher orders are also directed into the first order light band. Therefore, second order and higher light bands will not be observed when using a holographic diffraction grating. A calibration check on the diffraction gratings provided in this kit shows that the line spacing is approximately 950 lines/mm.
  • For more information on the laser principle, refer to Laser Theory, Publication No. 10426.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Asking questions and defining problems
Developing and using models
Using mathematics and computational thinking
Engaging in argument from evidence

Disciplinary Core Ideas

MS-PS4.B: Electromagnetic Radiation
HS-PS4.B: Electromagnetic Radiation

Crosscutting Concepts

Patterns
Systems and system models
Energy and matter

Performance Expectations

MS-PS4-2: Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
HS-PS4-1: Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
HS-PS4-3: Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.

Sample Data

Part 1. Light Interaction

{12766_Data_Table_1}
{12766_Data_Table_2}
{12766_Data_Table_3}
Part 2. Reflection
{12766_Data_Table_4}
Part 3. Refraction
{12766_Data_Table_5a}
{12766_Data_Table_5b}
{12766_Data_Table_6}
Part 4. Diffraction
{12766_Data_Table_7}
{12766_Data_Table_8}

Answers to Questions

  1. Which color filter transmitted the most light? Explain.

    The red filter transmitted the most light. The blue filter transmitted the least. The red filter looks red because it transmits mostly red-wavelength light. The green and blue filters have their colors because they transmit mostly green-wavelength and blue-wavelength light, respectively. The green and blue filters must also transmit a small amount of red light because some of the laser light was not blocked by the filter.

  2. Which colored paper reflected the most light? Explain.

    The red paper reflected the most red light because it appears red by reflecting red and strongly absorbing most of the other colors (blue and green).

  3. Explain why the blue and green strips of paper also reflected red light.

    The blue and green filters reflect mostly blue- and green-wavelength light, respectively. They must also reflect red-wavelength light to some degree because some of the red light from the laser pointer was reflected. However, under white light conditions, the blue- and green-wavelength reflections are stronger and mask any reflected red light.

  4. According to your observations of laser light traveling through a polarizing filter, is laser light polarized?

    Yes, laser light emitted from the laser pen is polarized because when the polarizer was turned appropriately, it either absorbed the laser light or transmitted it.

  5. In your own words, explain the law of reflection. (Optional: what mathematical equation describes the law of reflection?)

    When light reflects off a surface, the reflected light will travel with the same angle away from the surface as it did coming in.

    Equation 3: θi = θr; θi is the incident angle, θr is the reflected angle

  6. (Optional) Assume a person is standing in front of a vertical mirror. What is the minimum height the mirror must be in order for a person to see his or her entire reflection.

    In order for an individual to see their whole body while standing in front of a vertical mirror, the mirror must be at least half as tall as the person. It does not matter how far away from the mirror the individual is. Light reflects off a surface at the same angle from which it strikes the surface. Therefore, in order to see feet from a reflection, the light must reflect from the midpoint of the person’s height. If the mirror is any higher, and the reflected light from the feet would not reach the eyes. So, the smallest a mirror must be half as tall as the person.

  7. Snell’s law is given by:
    {12766_Answers_Figure_13}
    {12766_Answers_Equation_2}

    n1 = index of refraction of incident medium
    θ1 = incident angle of light beam (with respect to the vertical) at the media boundary
    n2 = index of refraction of exiting medium
    θ2 = exiting angle of light beam (with respect to the vertical) at the media boundary

    Use Equation 2, and the data from Experiment 5, to determine the index of refraction of water.
    nwater = nair sin θair /sin θwater
    nwater = (1.0) sin (30º)/sin (21.0º)
    nwater = 1.40
    Average value from the data in Data Table 5b: nwater = 1.35

  8. The accepted value for the index of refraction of water at 20º C is 1.333. How do your results compare with the accepted value?

    The results were close to the accepted value of the index of refraction of water.

  9. When total internal reflection first occurs (the critical angle), where does the transmitted beam go?

    The transmitted beam projects along the flat side of the dish and extends horizontal to the flat side. The angle of the refracted beam is 90º from the vertical.

  10. Using Snell’s law (Equation 2), and the critical angle measured in Experiment 6, determine the index of refraction of the plastic dish. Use the accepted value for the index of refraction of water found in Question 8.

    nwater sin θwater = nplastic sin θplastic
    nplastic = (1.333) sin (53º)/sin (90º)
    nplastic = 1.06

  11. Can total internal reflection occur when light travels from air into water? Why or why not?

    No, total internal reflection only occurs when the light travels from a medium of higher index of refraction into a medium of lower index of refraction. The light beam must refract away from the vertical in order for total internal reflection to be possible.

  12. From the data collected in Experiment 7, use Equation 1 (found in the Background section) to determine the wavelength of the laser light. The number of lines per millimeter of the diffraction grating is 950 lines/mm.

    sinθ= mλ/d
    λ= (1 mm/950) sin (37º)/1
    λ= 6.33 x10–4 mm = 633 nm

  13. Were the diffracted light lines brighter or dimmer than the center line?

    The diffracted lines were slightly dimmer than the center line.

  14. Why are the locations of the bright bands different when the light diffracts in air compared to when the light diffracts in water?

    The bright spot locations changed because their position depends on the wavelength of light that is diffracted. When light travels through water, the wavelength changes proportionally to the index of refraction of the water. In water, the light wavelength is shorter, so therefore the light is diffracted with a smaller angle compared to a longer wavelength light beam—the beam unaffected by the water.

  15. Determine the index of refraction of water using Equation 1, Equation 2 and the data collected in Experiment 8.

    [sin θa = mλa /d]/[sin θw = mλw/d]
    sin θa /sin θw = λaw = nw (index of refraction of water)
    nw = sin (37)/sin (28) = 1.28&

Teacher Handouts

12766_Teacher.pdf

Student Pages

Laser Pointer Education

Introduction

Use a simple pen laser pointer to observe and verify the “laws of light”—reflection, refraction and diffraction. Observe the properties of light that allow for the transmission of light down a fiber optic, and determine the wavelength of the laser beam using a diffraction grating.

Concepts

  • Reflection
  • Refraction
  • Diffraction
  • Interfearance

Background

Reflection and refraction of light are familiar properties of light. We use a mirror to see ourselves when we comb our hair. We wear eyeglasses to correct our vision. In this lab, the properties of reflected light will be verified and the index of refraction of water will be determined using Snell’s law. The combination of reflection and refraction will also be explored when the phenomenon of total internal reflection is observed.

Total internal reflection can occur when light travels from a medium with a high index of reflection to a medium with a low index of refraction. Fiber optic cables are the foundation of the “Information Super Highway” and they rely on total internal reflection to transmit communication signals (e.g., Internet, telephone, television) over incredibly long distances. The critical angle is the minimum angle (as measured from the normal, or vertical, line) at which light strikes a boundary surface (traveling from high index of refraction to low index of refraction) and all the light is completely reflected from the surface. No light transmits through the media boundary when light strikes this boundary at an angle greater than or equal to the critical angle. In this experiment, the measured critical angle will be used to determine the index of refraction of a plastic dish. Please refer to your physical science or physics textbooks for more information on the topics of reflection, refraction, and total internal reflection.

Diffraction is a property of light that is not commonly observed in everyday life. Diffraction occurs when light travels through thin slits. The light exiting the thin slit spreads out at an wide angle as if the light originated from the slit. The slit acts like a point source (see Figure 1).

{12766_Background_Figure_1}
If all the light is the same wavelength and phase, also known as coherent light, then an interference pattern will develop. We do not normally observe diffraction in our daily lives because most of the light we see is composed of all different wavelengths, intensities, polarization and phases. Laser light is coherent light, so when it strikes the thin slits of a diffraction grating an interference pattern will be observed. The pattern develops because the coherent light has to travel different distances to reach the same point. If the path lengths are just right, two light waves will reach the same point “in phase” and result in constructive interference, generating a bright spot of light. If the light pulses are “out of phase” when they reach the same point, the light waves destructively interfere and a dark region is created (see Figure 2).
{12766_Background_Figure_2}
The following equation can be used to predict the locations of the bright bands when coherent light passes through a diffraction grating.
{112766_Background_Equation_1}

λ is the wavelength of light
d is the width of the diffraction grating slits (reciprocal of lines/mm)
θ is the angle of the bright band from the normal
m is the order of the band

This equation will be used in Experiment 7 to determine the wavelength of the laser beam.

Materials

Beaker, 250-mL
Cap, black, with holes
Color filters, red, green and blue
Diffraction grating, holographic
Dish, semicircular
Folder, manila
Laser pointer, pen-type
Lens, cylindrical. 1" long, ¼" diameter
Mirror, flat
Mirror stand
Pen, permanent, black
Polarizing filter, 5 cm x 5 cm
Protracter Master Sheet
Ruler
Tape, transparent
Water

Safety Precautions

Do not aim the laser pointer directly into anyone’s eyes. The low-power, coherent light can cause damage to the sensitive retina and may lead to permanent blindness. Prevent stray laser light from projecting beyond the classroom to eliminate any unintentional exposure to the laser light. When reflecting the laser light between mirrors, it is best to do this low to the floor to keep the reflected laser light below “normal” eye level. For people with sensitive eyes it is recommended that dark, IR-protective, safety glasses be worn. Follow all other normal laboratory safety guidelines.

Procedure

Part 1. Light Interaction

Experiment 1. Color filters

  1. Obtain the three color filters: red, green and blue.
  2. Shine the laser beam through the green filter onto a white sheet of paper. Does the light transmit through the filter? How bright is the transmitted light? Record your observations in Data Table 1.
  3. Repeat step 2 for the other filters. Record all observations in Data Table 1.

Experiment 2. Colored objects

  1. Obtain the three sheets of paper colored red, green and blue.
  2. Shine the laser beam at the green strip of paper. Does any light reflect off the paper? How bright is the reflection? Record your observations in Data Table 2.
  3. Repeat step 5 for the blue and red paper sheets.
  4. Compare the reflected light intensity for the three colored paper sheets.

Experiment 3. Polarizer

  1. Obtain the polarizing filter.
  2. Hold the polarizer by its edge and place the polarizing filter in the path of the laser beam and project the beam onto a sheet of white paper. How bright is the beam that travels through the polarizer compared to the original beam? Record your observations in Data Table 3.
  3. Slowly rotate the laser pen 90 degrees. How does the transmitted beam intensity change? Does it become brighter? Dimmer? Record your observations in Data Table 3.
  4. Continue to rotate the laser pen 270° until a full circle has been completed. Record your observations in Data Table 3.

Part 2. Reflection

Experiment 4. Law of reflection

  1. Obtain the mirror, mirror stand, protractor sheet, black cap with holes, and the cylindrical lens.
  2. Insert the cylindrical lens through the holes in the side of the black cap (see Figure 3).
    {12766_Procedure_Figure_3}
  3. Place the black cap, with the inserted cylindrical lens, over the end of the laser pointer as shown in Figure 3.
  4. Turn on the laser pointer and observe the thinly spread light beam. A tightly spread out beam should appear as a “red light line” on the wall. When performing the following experiment, be sure to rotate the laser so that the red light line is vertical (the cylindrical lens will be horizontal).
  5. Place the mirror in the mirror support stand as shown in Figure 4.
    {12766_Procedure_Figure_4}
  6. Place the mirror in the center of the protractor sheet as shown in Figure 5. The mirror should be parallel with the horizontal line on the protractor sheet.
    {12766_Procedure_Figure_5}
  7. Aim the laser beam from a 30º angle (with respect to the vertical) directly at the center dot on the protractor sheet. Make sure to aim the light beam through both the 30º mark and the center dot of the protractor sheet.
  8. When the light beam is precisely aligned, measure the angle of the reflected light with respect to the vertical line. Record your measurement in Data Table 4.
  9. Repeat steps 18 and 19 for three different incident angles. Record the reflected angles in Data Table 4.

Part 3. Refraction

Experiment 5. Law of refraction (Snell’s law)

  1. Obtain the semicircular dish, ruler, black permanent marking pen, protractor sheet, black cap with holes, cylindrical lens, 250-mL beaker, manila folder and about 100 mL of water.
  2. Measure the length of the flat side of the semicircular dish.
  3. Divide the length by 2 to determine the midpoint of the flat side of the dish.
  4. Measure the midpoint on the flat side of the semicircular dish and make a thin vertical line with the marking pen. This marked line will be the laser beam target on the dish.
  5. Fill the dish with water and place the dish on top of the protractor sheet so that the midpoint mark on the dish is lined up with the center point of the protractor sheet as shown in Figure 6.
    {12766_Procedure_Figure_6}
  6. If the black cap and cylindrical lens are not already on the laser pen, refer to Procedure steps 13–15.
  7. Stand the manila folder vertically about 10–15 cm from the round side of the semicircular dish as shown in Figure 6. Support the manila folder vertically with a 250-mL beaker. The manila folder should be parallel to the flat side of the dish and will act as a backstop for the light.
  8. Start at 30º. Aim the laser directly at the center of the protractor sheet and the midpoint of the flat side of the semicircular dish. Make sure the spread-out light line is vertical.
  9. Since the light beam is projecting vertically, the “original” light beam will strike the white sheet of paper along with the beam that travels through the water. Compare the position on the manila folder of the “original,” unaltered light beam to the light beam that traveled through the water. Record your observations of the deflection of the laser beam in Data Table 5a.
  10. Record the incident angle, with respect to the vertical, at which the beam enters the water in Data Table 5b. (A good check: If the beam is striking the dish through the midpoint, the original beam should travel in a straight line through the same angle on the opposite side of the dish.)
  11. Record the refracted angle, with respect to the vertical, at which the beam travels through the water in Data Table 5b.
  12. Repeat steps 30 and 31 for three more incident angles. Record the measurements in Data Table 5b.

Experiment 6. Measuring the critical angle for total internal reflection

  1. Obtain the semicircular dish and about 100 mL of water.
  2. If there is not already a mark at the midpoint of the flat side of the semi-circular dish, perform Procedure steps 22–24.
  3. Fill the dish with water and place it on top of the protractor sheet so that the midpoint is lined up with the center of the protractor sheet, as shown in Figure 6.
  4. If the black cap and cylindrical lens are not already on the laser pen, refer to Procedure steps 13–15.
  5. From the curved side of the dish, aim the laser beam through the water towards the pen mark on the flat side of the dish. Record your observations in Data Table 6.
  6. Slowly change the angle of the incident light beam reflecting off the flat side of the dish starting at 20° and moving towards 90° from the vertical. Be sure to keep the laser beam aimed at the mark on the flat side of the dish (the center of the protractor sheet). Is there a minimum angle at which no light transmits through the flat side of the dish? Record any observations in Data Table 6.
  7. Slowly change the angle of the incident light on the flat side of the dish until the transmitted beam just disappears and all the light is completely reflected (no light transmits through the flat side of the dish).
  8. Record the incident angle of the light beam that first causes “total internal reflection.” Remember to keep the laser beam aimed at the midpoint of the flat side of the dish (and therefore at the center of the protractor sheet).
  9. What happens to the light if the angle is larger than this “critical” angle? Record your observations in Data Table 6.

Part 4. Diffraction

Experiment 7. Measure the wavelength of the laser pen via diffraction

  1. Obtain the holographic diffraction grating, mirror support stand, black cap with holes, cylindrical lens and protractor sheet.
  2. If the black cap and cylindrical lens are not already on the laser pen, refer to Procedure steps 13–15.
  3. Set the holographic diffraction grating in the mirror support stand as shown in Figure 4.
  4. Place the diffraction grating at the center of the protractor sheet so that it is parallel to the horizontal line on the sheet.
  5. Shine the laser through the diffraction grating and observe the split beams.
  6. Adjust the direction of the laser beam so that the center beam runs perpendicularly through the 0º and 90º marks on the protractor sheet, and the side beams are equidistant from the center beam.
  7. Record the angles of the sideband beams in Data Table 7. (It may be easier to read the angles of the bright bands if the laser pointer is raised slightly. The laser pointer should remain parallel to the tabletop. The center beam should still run through the 0º and 90º marks on the protractor sheet.)

Experiment 8. Observing diffraction

  1. Obtain the holographic diffraction grating, transparent tape, semicircular dish, black cap with holes, cylindrical lens, a 250-mL beaker, manila folder and about 100 mL of water.
  2. If the black cap and cylindrical lens are not already on the laser pen, refer to Procedure steps 13–15.
  3. If the midpoint has not already been drawn on the semicircular dish, refer to Procedure steps 22–24.
  4. Tape the holographic diffraction grating to the outside of the flat side of the semicircular dish (Figure 7).
    {12766_Procedure_Figure_7}
  5. Fill the dish with water and place the dish on top of the protractor sheet so that the midpoint mark on the dish is lined up with the center point of the protractor sheet as shown in Figure 6.
  6. Stand the manila folder vertically about 10–15 cm from the round side of the semicircular dish as shown in Figure 6. Support the manila folder vertically with a 250-mL beaker. The manila folder should be parallel to the flat side of the dish and will act as a backstop for the light.
  7. Shine the laser beam at the diffraction grating through the midpoint on the dish so that the light line is vertical and goes through the water and above the dish. The center beam should run through the 90° line on the protractor sheet. You should observe three center bands hitting the backstop and two sets of bright bands to the left and right of the center beam.
  8. Move the laser pointer so that the beam strikes the diffraction grating perpendicularly and at the midpoint of the dish so that the position of the three center bands come together to form a vertical line in the middle of the backstop. When all three center bands form a vertical line, the laser beam is hitting the diffraction grating perpendicularly at the midpoint of the flat side of the dish.
  9. With the center bands lined up vertically, record the angles of the two wide-angle bright lines and the two small-angle bright lines in Data Table 8. (It may be easier to read the angles of the bright bands if the laser pointer is raised slightly. The laser pointer should remain parallel to the tabletop.)
  10. All materials should be returned to the instructor for proper storage.

Student Worksheet PDF

12766_Student1.pdf

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