1) Albert A. Michelson (1852-1931) modified a method used to measure the speed of light that was designed originally by Jean Foucault. In 1879 Michelson used a rorating mirror anc accurately measured the speed of light.

The light is emitted by a source S , reflected off an orthogal mirror A and travels to a 2nd mirror very very far away. The mirrors were place on the top of 2 CAlilfornia mountains, 35km apart. When it comes back it is, ounce again, bounced off the rotating mirror C and seen by the observer. While the light travels, the mirror B moved to C. The time required for B to move to C was just one eigth of the timed needed for one revolution of the mirror. Starting from rest, the speed of the motor was adjusted until the light reflected from C was clearly observed through the telescope.
A) Why the time take by the light is one eight of a revolution?

B) The octogonal mirros makes 32,000 rpm = 32,000 rev/minute. WHat time is required for 1 revolution?

C) What time is needed for the mirror to move from B to C ?

D) Find the speed of light (remember, the montains are 35km apart) in km/s ? in m/s ?

E) Galileo tried to measure the speed of light. Without sucess. Describe his experiment.


F) In 1887Michaelson tried another experiment with Morley. He wanted to show the existence of the ether. Light was thought to be a waveand as any wave it was thought to be traveling in a medium. They called this medium ether. Its was supposed to be very light as light was really fast and it was supposed to offer no resistance to the motion of the EArth. Light was due to the waving of this medium. It turned out that he proved the oposite. There was no ________ . So for the light to propagate , what is waving ?
_____________. THe experiment also showed that the speed of light does not depend on ___________ or ______________.
THis was one of the principle of the special relativity of Einstein. (more )
more about this experiment

2)sound channel in the atmosphere

You just have learned that waves bend and undergo total reflection when going from a slow layer to a fast layer. Waves bend away from the fast layer (remember mirage?). They bounce off the fast layer. (see trapped light, critical angle)

 Sound is a wave. Sound wave moves faster in a warm air than in a cold air. In a warm air the molecules have larger speeds and the speed of the sound is the speed of the air molecules. It also moves faster when the pressure increases (in a liquid or gas). The molecules are closer to each other and the sound is more easily transferred.

HAve you ever heard of Roswell , New Mexico (the UFO story and the allien) ? The file is now declassified. How can you relate Roswell event to refraction?

hint:In the atmosphere, from EArth's surface to the ozone layer, the air is warm (above the ground), cold as you go higher in the atmosphere then warm in the ozone layer (warm/cold/warm). The temperature decreases with the height then increases again.  Also, during the cold war, Americans were trying to "listen"to the Russian nuclear tests. At the time of the Roswell event, microphones were huge disks and the army was mastering the hot balloon technology.

3) Sound channel in the ocean

How come whales are able to sing to each other from one hemisphere of the planet to the other ? (that's pretty far)

hint: In the ocean, from the surface to down in the ocean, the speed of sounds first decreases (temperature decreases) then increases again (the pressure increases) and then decreases again (temperature keep decreasing).

4) During World war 2, American pilots whose planes were shot above the ocean, but who were still alive, could use an empty sphere to let the marine knows their position. ANy idea ? It has to do with the sound channel and refraction again. (see the example with whales)

5) In the morning you can hear from very far. Do know why? In the evening, it is usually the opposite. Why?

hint: In the morning air above the ground is cold and the air higher is warm. In the evening, the ground has absorbed the radiation of the Sun and is now emitting IR so it is warmer than the air higher. remember wave bounce off fast layer.