University of Virginia
Physics Department

## Waves and Particles

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9

Objectives

Students will

• understand that light sometimes behaves as a particle and sometimes as a wave;
• do experiments that demonstrate light must behave as a wave;
• use diffraction gratings to analyze various sources of light;
• understand that white light is a broad spectrum of many colors;
• learn that elements produce characteristic light spectra;
• understand the importance of diffraction.

Motivation for Learning

1) Laser light in a Water Tank

Material List:

• aquarium (fish tank, 5 gallon size is ideal)
• water · laser pointer (He-Ne, red/orange in color)
• powdered milk (coffee creamer)
• white poster board, at least 12" on a side to stretch across the top of the fish tank

Procedure

This is a very nice set up to demonstrate both reflection and refraction.

1. Fill the aquarium (clear glass) almost full of water; leave about 5 cm unfilled.
2. Mix in less than a teaspoon of powdered coffee creamer and stir. You may have to do this in stages to obtain the optimum amount, but if you put in too much, you will have to start over.
3. Shine the laser pointer from the outside of one end of the aquarium from near the bottom pointing so the light will come out through the surface of the water (see photo below). You should be able to see the red path of the laser as it passes through the water. You will not be able to easily see laser light pass through clear water, but the powder adds larger masses that will scatter the laser light so you can see it. You should be able to see the light reflect off the top water surface. If you shine the laser pointer towards the side of the aquarium, you may be able to see light reflected off the glass side.
4. You will not be able to see light coming out of the aquarium into air, because air molecules are too small to effectively scatter visible light. However, you can place the white poster board over the top and sides of the aquarium and see the laser light shine on the white board.
5. If the laser light is quickly attenuated inside the aquarium, you have put too much coffee creamer in the water. Start over. If you can't see the light or if you can barely see it, then you need to put more creamer in and stir. It may take some practice to get this just right. Measure what you put in so you can reproduce the amount later. This photo was produced using a more powerful laser than a laser pointer, so the light could easily be seen in the 5-gallon fish tank for the photo. We used less than one teaspoon of coffee creamer. The laser can be seen in the lower right of the photo (red spot). The path of the laser light cannot be seen until after it enters the water. The light moves to the left and is totally internally reflected at the water surface. Note the straight lines that the laser light makes inside the aquarium. The fish tank can be seen as outlined by the cloudy area in the photo.
6. Shine the laser light so that it reflects off the water surface (from inside the water) at different angles. Use the poster board to see if light is being transmitted through the water/air interface (that is, is light being refracted?). You should easily be able to see the laser light reflect off the glass surface where it enters the aquarium (use poster board).
7. You should be able to find an angle (called the critical angle) where, as the angle with the water surface gets smaller, all the light will be reflected from the water surface back into the aquarium and none will be refracted out into the air. This angle should be about 40 degrees from the water surface for water and air. Smaller angles should have the light totally reflected back into the water. You will have to move the poster around on top of the aquarium to see the red spot, because the direction of the refracted moves dramatically with entrance angle.

2) Laser pointer through chalk dust

Materials:

• Laser pointer mounted in firm holder to shine long distance across room
• Chalk eraser and chalk (or anything similar to make small airborne particles, like talcum powder, or even water in a water sprayer)

Procedure:

Set up the laser pointer in some holder (see photos below for activities using laser pointer and slits) so that it shines several feet across the room. Be careful not to shine it in anyone's eyes and point it away from anyone. Rub the chalk on two erasers so they are full of chalk. Then holding one eraser in each hand, clap the two erasers together right above where you think the laser light path should be. You should be able to see the light reflect off the large dust particles. The probability of light reflecting off air molecules is very small, because of the small size of the molecules. However, the reflection probability rises sharply for larger particles like chalk dust and talcum powder. Notice how straight the laser light path is.

3) Splitting Visible Light

Materials:

• Diffraction grating, large if possible
• Projector screen in a dark room
• Opaque paper with a slit, about 2 mm wide and 10 cm high to fit on overhead projector

Procedure:

Using opaque paper large enough to cover the glass bed surface of the overhead projector, use a knife or razor blade to cut a narrow slit about 2 mm wide and 10 cm high. The exact dimensions are not important. You can even bring two pieces of paper close together to make the narrow slit. See the figure.

The room should be dark to see the splitting of the light into the rainbow colors. This is a typical continuum spectrum of white light. A holographic diffraction grating placed on cardboard is ideal to cover the lens of the projector. If this is not available use a small diffraction grating mounted on a 35 mm cardboard slide. Be careful not to touch the fragile diffracting grating. (You can get diffraction gratings on 35 mm slide holders from most science suppliers. You can also purchase sheets of holographic diffraction gratings (and slides) from Learning Technologies, Inc., 59 Walden Street, Cambridge, MA 02140, 800-537-8703. (They can be cut up into pieces.)

By replacing the diffraction grating with a prism, you can show the students that diffraction gratings are similar to prisms in that both of them separate light out into different wavelengths. Diffraction gratings are much more useful in this process. You should be able to see the colorful spectrum to the left and right of the center bright narrow slit on the screen when using the grating.

This phenomenon can only be understood if light behaves as a wave. The particle theory of light cannot explain this. However, the wave theory of light can be used to explain the particle phenomena that we saw earlier.

Background Information

One of the great lessons of physics is that light sometimes behaves as a particle and sometimes as a wave. Light cannot both behave as a particle and a wave at the same time. Because this is a subject of quantum physics, we will restrict ourselves to performing a few activities to understand that light must be described in general as a wave. We will point out those areas in which the particle concept works. We will not perform any experiments, such as the photoelectric effect, that requires a particle concept to understand. When we are expressing light as a particle, we call it a photon. A photon is a bundle of electromagnetic energy. We will only study electromagnetic radiation in this activity in the visible region. See the activity on the Electromagnetic Spectrum for a fuller discussion of electromagnetic radiation.

In our everyday life, we have significant evidence that light travels in a straight line as does a particle. This study of light is called ray optics, because the particles of light are treated like a ray. The popular ray guns of science fiction originate from this concept. Light does not travel around corners. We will do a couple of demos and activities here to demonstrate that light seems to travel as a particle. These include the fish tank demo that we did in Total Internal Reflection, which can be repeated here. We can clap erasers saturated with chalk dust above the path of a laser pointer to see that light travels like a particle in a straight line.

However, we will do other experiments here that cannot possibly be explained if light consists of particles. We will see that single, double, and multiple slits seem to separate and spread out light. We can only understand this if light acts as a wave. There are many experiments that confirm this, and scientists now understand that, in many cases, the particle theory of light is only a special case of the wave theory. The wave theory of light is correct; it can explain all the phenomena we are going to study here. In some special cases where the apertures through which light travels is much greater than the wavelength of the light, the particle theory can also explain what we observe, but we must understand this is only an approximation. Consider the following phenomenon: light of wavelength l passes through an opening of width a.

q = l/a

When l is on the order of size a, diffraction effects will be seen. That is, the light waves will be bent as they pass through the narrow aperture. Remember that visible light waves have a wavelength on the order of 500 nm, so these slits are very narrow! For larger slits, the angle q is very small, and in order to see the effects of diffraction, one has to look very far away. What one sees are alternating bright and dark places. When waves of any kind (water, sound, light) meet they interact. One possibility is that they can cancel each other out. This is called destructive interference and is seen as the dark areas between the bright spots. The components of the waves can also add together. This is called constructive interference and it produces bright spots. Light is a wave. When light passes through any aperture, we can consider the aperture as a source of new waves or wavelets. These waves move out and can interfere with each other anywhere. Interference can occur whether the light waves leave from a single slit, double slit, or multiple slits. Interference and diffraction are similar. Interference occurs because individual waves emanating from the narrow aperture add together positively and negatively (or interfere with each other) to produce bright and dark regions. These can normally only be seen in a dark room. We present here a list of websites, which may be useful in learning about this subject.

http://www.jetcity.com/~science/homeschool_science/light.html

Part I

1. Yes, the holes are lined up in a straight line.
2. The light does not pass through when one of the cards is moved out of line.
3. Yes, light travels in a straight line.
4. You should not see light come all the way through the straw after you bend it.

Part II

1. No.
2. The angle q ~l/a = 620 x 10 -9 m/2 x 10 -3 m = 310 x 10 -6 rad = 0.02°. The only way we can observe this is to look very far away. If we observe at a distance of 4 m, we would expect to see an effect about 1 mm from the central spot.

Part III

1. The angle at which the light bends is proportional to the wavelength of the light. Red light has a longer wavelength than blue light, so red light is bent more. It thus is located at a larger angle from the central spot.
2. A rainbow of continuous colors is seen with the open light bulb. With the various colored filters, most of the colors are blocked except for the filter color.
3. The lightest elements, hydrogen and helium, give the simplest spectrum. Hydrogen produces a clearly discernible violet, blue-green, and violet bands. Helium produces violet, blue, blue-green, yellow, and red. Neon has several bands, mostly in the yellow-red end of the spectrum which is characteristic of the neon signs.
4. Every element will have a characteristic set of colored bands when viewed through a diffraction grating. These colors depend on the electronic structure of the element. The hydrogen spectrum is the simplest, because it has the simplest electronic structure.
5. We have to excite the elements to higher energy levels. The power supply does this. When the electrons "fall back" to a lower energy level, a photon is emitted that has a characteristic wavelength. We are only able to see the wavelengths in the visible spectrum. In general, there are many other spectral lines that we cannot see that are in the ultraviolet and infrared regions.
6. You would see a complicated spectrum that is characteristic of the element mercury. You would see violet, blue, blue-green, green, yellow, and orange lines.

Part IV

1. The students should see a rainbow of colors, because the CD is acting as a diffraction grating.
2. The CD in this case is called a reflective diffraction grating.
3. The colored filters would absorb light except for the color seen. You would expect to see many colors in white light missing from the screen.

Part V

1. Squeezing the two pencils together makes the slit narrower. This should have the effect of increasing the angle of light diffraction so that the light spectrum moves apart.
2. The line of light should become vertical rather than horizontal. You should still be able to observe the colored spectrum.
3. Each of these objects, especially the diffraction grating, will act like slits and you should see colored spectra.
4. The colored filters would absorb light except for the color seen. You would expect to see many colors in white light missing from the screen.

### Student Activity

I) Does Light Travel in a Straight Line?

Materials

• flashlight
• 4 - 3" x 5" index cards
• means of supporting the index cards, or simply fold the index card so that it will stand on edge
• flexible soda straw

Procedure

1. Punch a small hole in each of the index cards at precisely the same position.
2. Stick each card into a white support to hold the card upright, or make a fold so that the card will stand upright.
3. Place the cards about 15 cm apart with holes in a straight line.
4. Shine the flashlight so that the light travels through the hole in each card.
5. Move one of the cards a little and observe if the light passes through.
6. Now try looking through the flexible straw while it's straight at a light source at eye level.
7. Bend the straw and look at the same source.

1. Look down the cards when the light is passing through all the cards. Are the holes lined up in a straight line?
2. What happened when you moved one of the cards a little?
3. Can we conclude that light travels in a straight line?
4. Did you see light come all the way through the straw after you bent it?

Materials:

• sharp knife or single sided razor blade
• black or opaque paper (or poster board)
• laser pointer mounted firmly in place
• projector screen
• dark room
• diffraction grating

Procedure

1. Use a sharp knife or a single sided razor blade to make a clean slit in the opaque paper. The height of the slit is not important, maybe 2 to 5 cm high. Put something (like cardboard or many sheets of paper) underneath the paper so you can make a clean cut and not harm the underneath surface. Then move over a few cm. Take two single sided razor blades and place a piece of paper between the blades. Then use the blades to make a clean double slit with the slits very close together. The paper between the blades serves to make two slits. Be careful not to rip the paper between the slits. You may need to go over the cut several times to make sure the cuts are clean and the two slits are visible.
2. We will be shining the laser light from the laser pointer on several items to see if we can see diffraction effects. REMEMBER NEVER SHINE THE LASER ON ANYONE ELSE AND PAY SPECIAL ATTENTION TO NEVER SHINE IT IN ANYONE'S EYES. Read here to learn more about Laser Pointer Safety
3. See the photo below to see how to mount the laser pointer. You can also simply tape the laser pointer to a book. Use a binder clip to turn on the laser pointer. Most of them have a clip or button that needs to be pushed closed. We used a wire with alligator clips to turn on the laser pointer in the photo.
4. First, mount the razor blade in front of the laser beam so the laser light shines on the edge of the blade. Place the screen several meters away and look for dark and light spots going out from both sides of the bright central spot. These fringes or interference patterns will probably be curved and can only be seen in a dark room. This phenomenon is due to diffraction and is a wave effect.
5. Next, mount the opaque piece of paper such that the laser shines on the single slit. Look for diffraction effects on the screen. You may want to move the paper around a little to see the best result. If you don't see anything, make sure you do have a clean, open slit. If not, go back and go through the slit opening again with the blade. This result is called the single slit diffraction pattern. It is difficult to see because only a small amount of light goes through the single slit.
6. Next, move the paper over so the laser shines through the double slit. This interference pattern should be sharper on the screen and somewhat brighter because light is going through two slits. You should actually see dots.
7. Now place a commercial diffraction grating in front of the laser. You will see bright dots. The diffraction grating has 1000s of lines per inch. Each line on the grating acts like the single slit, but the many slits interfere together and allow much more light to pass through.
8. Now shine the laser on a single hair. You can use human hair and stretch it out and tape it between a hole in cardboard. Shine the laser on the hair, and you will see interference patterns similar to the double slit. The hair intercepts the light making it seem like two narrow slits!

1. Can you think of a way to explain these phenomena using the particle theory of light?
2. If the wavelength of the laser light is 620 nm and the single slit width is 2 mm, what angle q would you expect? How can we observe the effects of such a small angle for the single slit?

III) Diffraction Gratings

Materials List:

• diffraction gratings, one for each student (they can be obtained from science suppliers like CENCO and Edmunds mounted on a 2" x 2" card. A set of 25 cards cost about \$25). You can also purchase simple spectroscopes that consist of a tube with a diffraction grating in one end and a slit in the other end.
• incandescent open bulb
• color filters
• gas discharge tubes of various elements with power supply (available from most science suppliers; CENCO has them)

Procedure

1. Every student receives a diffraction grating. Be very careful with them and DO NOT TOUCH THE GRATING! IT WILL BE RUINED IF YOU DO.
2. Set up an incandescent bulb in the middle of the front of the room. Each student should observe it through the grating. A rainbow of colors should be seen.
3. Place, one at a time, a color filter in front of the light bulb and observe.
4. Now use the gas discharge tubes with the power supply and look at the light through the diffraction grating. Write down what you see for each tube. Each of the tubes will be for a specific element. Common tubes are hydrogen, helium, neon, and mercury. Tubes containing air, carbon dioxide, and water vapor are also available.

1. Can you explain why the red light appears to be further away from the central spot than the blue light?
2. What is the difference between observing the open light bulb and the light bulb with colored filters in front?
3. For which of the spectral tubes do you tend to see the least number of colored lines?
4. Can you think of a way to identify gaseous elements?
5. Why do we need the power supply to see the light from the tubes?
6. What would happen if you took a diffraction grating out at night and looked at a mercury street lamp?

IV) CDs and Light

Materials List:

• audio CD
• flashlight with a narrow beam like a mini MagLite
• projection screen

Procedure

1. Each group should be given a CD. It can be an audio CD or one containing computer software. Make sure it is old and not needed.
2. Use a flashlight with a narrow beam and shine at an angle onto the CD in a dark room. Observe the reflected light on a white screen or white board. What do you observe?

1. Describe what you see on the screen?
2. The diffraction gratings that we look through are called transmission diffraction gratings. What do you surmise this process is called?
3. If you put colored filters between the MagLite and the CD, what would you expect to see on the screen? Try it if you have time.

V) A Simple Diffraction Experiment
This experiment is adapted from one at the Exploratorium (see http://www.exploratorium.edu/snacks/diffraction.html)

Materials:

• Mini-Maglite flashlight
• Two new pencils
• Thin tape

Procedure

1. Wrap one turn of thin tape around one of the pencils just below the eraser.
2. We will only want to use the light bulb of the Mini-Maglite, so unscrew the top of the flashlight and turn the light on. The light should be about a meter in front of you.
3. Hold the two pencils vertically, side-by-side and close together. The tape will form a thin slit between the pencils through which you will look at the light bulb. Hold the pencils just in front of your eye, maybe 2-3 cm.
4. What do you observe? You should be able to see a continuous rainbow spectrum.
5. Squeeze the pencils together tightly. How does the spectrum change?
6. Rotate the pencils 90° while continuing to look at the spectra.

1. What does squeezing the two pencils together do? Why does it affect what you see?
2. What did you see when you rotated the pencils 90°?
3. What would you see if you looked at the light bulb through various objects like
a) a feather?
b) metal screen?
c) diffraction grating?
d) piece of cloth?
Rotate each of them while observing.

Extensions

A nice extension to Activity II is to let the laser pointer shine on a commercial slit apparatus that has single slits, double slits, and diffraction gratings of different sizes. You can obtain these devices from science suppliers like CENCO (800-262-3626) and Edmunds (609-573-6250). There are several other sources of light that can be observed with diffraction gratings:
1) Neon bulbs can be obtained in specialty lighting stores.
2) GrowLuxă fluorescent lamps (with many bands) can be obtained from gardening supply stores.
3) LEDs of various colors can be obtained from Radio Shack. They must be wired with a current limiting resistor. LEDs are available in red, yellow, and green; they will emit strongly in those colors, but also weakly in others.
4) Blacklight fluorescent lamps will produce violet, green, and yellow lines.
5) Color white paper with fluorescent crayons and illuminate with UV light. The various colored crayons will emit different lines.

Students with Special Needs

Some students will need assistance in several aspects of this activity. Be careful with handling the laser pointer. Safety is very important in using the laser pointer. Teachers may want to do the laser pointer activity as a demonstration. To see the spectra with the diffraction gratings, the students will need to look off to the side; some students will need help.

Click here for further information on laboratories with students with special needs.

Assessment

The students should each write down what they observe in these activities on a Data Sheet and discuss their answers to the questions. It is okay for the students to discuss the questions and answers, but the students should write out the answers in their own words.