Try This Experiment!
How Far Apart are the Data Tracks are on a Compact Disc?
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NanoStuff
Nanosecond - A billionth of a second The fastest computers can do a billion things per second, so each thing takes a nanosecond.
Nanometer - A billionth of a meter A human hair is 10,000 nanometers wide and the period on this page is about 500,000 nanometers across!
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You have read that the data tracks on a CD are 1.5 millionths of a meter apart. How do they know that? After all, you cannot measure something that small with a ruler! Can you? In this experiment we will show you how to measure that spacing with a ruler and a laser.
What you will need for this experiment
- Laser pointer - You will need to know the wavelength of the laser, which should be written on the side. Sometimes a range of values will be listed; so, you should take the average or the value halfway between the two. The wavelength will be in nanometers (nm).
- CD (Any CD will do)
- Ruler with metric scale
- Tape measure
- Books for holders
- Rubber bands to hold the books tightly
- White paper for a screen
- A pencil to prop up the laser pointer
- Masking tape
Performing the Experiment
Note - This experiment should be done with adult supervision! Never look at the laser light directly with your eyes. It could damage your eyesight permanently!
- Place the CD in a book so that it shows above the pages.
- Wrap a rubber band around the book to hold CD in place (see Figure 1 and 2).
- Place the laser pointer on the top of a stack of books.
- Use a pencil to prop it up at an angle; the angle between the pointer and the CD should be small and the laser should be placed so that it is at a right angle to the CD.
- Tape a piece of white paper to the wall to use as a screen.
- Turn out the room lights.
- Turn on the laser pointer; if your pointer does not have a button to keep it on continuously, then place a piece of tape over the "on" button. (Note - To prevent damage to your eyesight, do not look directly at the beam!).
- Now you should see something cool! When the laser beam is on, you should see a bright central spot on the screen with two smaller arcs on either side (Figure 2B); most likely, the arcs will be at an angle because the CD is round and will not be the same on both sides of the central spot because the laser beam is at an angle to the CD.
- With your ruler, measure the distance from the CD to the screen in centimeters. Call this distance Length.
- Measure the distance from the central spot to each of the arcs on either side (arc1, arc2); both distances should be measured in centimeters.
- Add arc1 plus arc2 and divide the sum by 2. Call this Arc Mean.
- Use the online calculator with your values of laser wavelength, Length, and Arc Mean to calculate the distance between the data tracks on the CD.
What you will get from the online calculator is the distance in microns (i.e. millionths of a meter) between the data tracks on your CD. How close is it to the average value of 1.5 microns that you read in the article? Try these experiments:
- Repeat the experiment with another CD. Do you get about the same spacing (within 0.1 or 0.2 microns)?
- What happens if you move the CD closer to the wall? Does it affect your calculations?
- What happens if you move the CD farther away from the wall? Does it affect your calculation?
- What actually happened?
Light is actually a wave of energy. As the waves of light from the laser beam were reflected off the tracks of the CD, they spread out. and interfered with each other (i.e. diffracted) when they hit the screen. Parts of the waves reinforced each other (i.e. added together), while other parts cancelled with each other to produce the pattern on the screen (Figure 2B). This pattern was first predicted by Christian Huygens in the 18th century and later demonstrated experimentally by Thomas Young in 1803; Young's results were taken as the first evidence supporting the wave theory of light proposed by Huygens.
Related Links:
Related Books:
"Conceptual Physics," Hewitt, Paul G., (1999) Third Edition, Scott-Foresman-Addison-Wesley, Inc., Menlo Park, CA, pp 487-490.
"Holt Physics," Serway, Raymond A, and Jerry S. Faughn, (1999) Holt, Rinehart, and Winston, Austin, TX, pp 598-613.
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