Unformatted text preview: palms under each end and the backs of your hands resting on the table, align the rod with the
beam.) While one partner is aligning everything, the other partner can press the Run/Stop button when the
signal is satisfactory. Pressing Run/Stop will “freeze” the trace so that you can do the analysis without
needing to hold the rod in place for the entire time.
3. In a calculation similar to that you did for the speed of light in air, determine the speed of light in acrylic.
Calculate the index of refraction n from this and write both in hand-in sheet.
4. Turn off the laser (pushbutton) and the detector (lever switch).
Questions (Write in hand-in sheet)
1. In air, n=1.0003. Can materials have n<1? Why or why not?
2. In 1638, Galileo described a seemingly straightforward experiment to measure the speed of light.
Galileo and an assistant, each carrying a shuttered lantern, would position themselves on the tops of two
mountains separated by a great distance. Galileo would open his shutter; the assistant would then open his
shutter upon seeing the light from Galileo's lantern; Galileo would then close his shutter upon seeing the light from his assistant's lantern. The time difference between Galileo's opening and closing his shutter
could then be used to determine the speed of light. If the mountain tops are separated by 1 mile, how long
will it take light to travel the round trip between them? Based on your answer, how successful do you think
Galileo was in determining the speed of light? Explain.
3. Many years ago it was postulated that light waves traveled in a medium called the “aether” that pervaded
the whole Universe (much like water waves need water, and sound waves need air as their respective
media). If such medium did in fact exist, what extra precautions would you take to obtain an accurate
reading for the speed of light considering the fact that your laboratory would be moving with respect to the
4. Were you able to completely eliminate the reflection pulse by using a terminator? If not, would a greater
selection of terminating resistors have helped you? Why or why not?
5. The reflected pulse has the same polarity as the original pulse when there is no terminator in place, yet it
has the opposite polarity when you use a paper clip to “terminate” the cable – explain this in Mechanical
terms, i.e., using a wave pulse along a string which is anchored on one end and may or may not be anchored
on the other end.
6. Companies that manufacture photographic lenses often put multi-coatings on their lenses. These are
thin, transparent films of slightly different indices of refraction. The idea here is to try to match the
“impedance” of glass (n=1.5) to air (1.0) so that less light is reflected back from the lens (that's why lenses
always look dark – look at your camera's lens). Detail how you would place 10 such coatings on the lens
surface, and give an example of what indices of refraction they should each have....
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- Summer '09
- Light, Coaxial cable, pulse, hand-in sheet