How Is Light Affected When Passing Through Water?


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"when passing through" water is a toughy, because simply passing through water won't do a whole lot to light except make it a bit dimmer.
Passing INTO and OUT OF water, however, is a different story.
Water is about 800 times as dense as air, and the light actually travels more slowly in water (about 140,000 miles per second) as in air (about 186,000 miles per second). That means that when light passes from air into water, it bends. It also bends when it passes from water back into the air, though it bends the opposite direction. Weird, huh?
The bending of light due to differences in density is called refraction, and it's how the lenses of a telescope make distant planets seem close. Lenses refract, mirrors reflect.
The direction that the light diffracts (left or right) depends on the angle of the light with respect to the surface of the water.
Here's a very good web-site that shows, at the bottom, this difference:
Something to note: Different layers of air have slightly different densities, so the light can diffract a little bit as it goes through the atmosphere. I remember reading somewhere that as the sunlight enters the atmosphere, it bends so much that if you see the sun just touching the horizon, the ACTUAL sun is already completely BELOW the horizon, but the light bending in the atmosphere makes it LOOK like it isn't.

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