- (a) Normally the sky appears blue, and this is because the air particles are large enough to form pretty good antennas for blue light (which has the shortest wavelength in the visible spectrum), but not large enough to serve as an antenna for the longer wavelengths of visible light. However, when the air contains a lot of water droplets, which are quite large even compared to the wavelength of red light, they serve as good antennas for all colors of light. Since all colors are then scattered, the sky appears white.
(b) On a very clear day, only blue light will be scattered, and not even too much of that. So the sky appears dark blue.
(c) The Moon has no atmosphere, so no light is scattered. Thus the sky appears black--you are looking directly at outer space.
(d) There is a change in the speed of light when light goes from air to water (same thing as a change in the index of refraction). Some light is transmitted at the interface (and refracted in the process) and some is reflected.
(e) Light always bends (refracts) when it changes speed, as it does when it moves from colder air to hotter air. Light moves faster in hotter air, since there are less air molecules to slow it down.
- Looking through a small hole (by squinting, or by actually making a small hole in a sheet of paper) makes your eye act more like a pinhole camera, in which case the "circle of confusion" on your retina is smaller and a greater range of objects will appear acceptably in focus. The disadvantage is that the image is dimmer.
- Since your ear can hear up to about 20,000 Hertz (cycles per second), we must sample (take a measurement of the signal) at least that many times per second (actually at least twice that many times per second). If we take fewer samples, the music will not faithfully be reproduced.