University of Virginia
Physics Department

## Waves and Particles

A Physical Science Activity

### 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 card so it has an edge to stand up on
• 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 fold the card to make it 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. put in photo of person cutting slit with razor blade
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. Put in photo of laser shining on razor blade.
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. Put in photo of laser shining on double slit
7. Now place a commercial diffraction grating in front of the laser. Now 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! Put in photo of laser shining on hair

1. Can you think of a way to explain these phenomenon 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, 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.