Describe what happens to a beam of light when it enters water or Plexiglas from air.
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The phenomenon you are asking about is called refraction. Refraction is what happens to light as a result of traveling through a different medium, such as from air to water, or from air to plexiglass. The speed of light slows down when encountering more dense material, which water and plexiglass represent. Since the speed of light slows down, the objects the light reflects from appear differently. This may be demonstrated by filling a clear glass about half full of water. Stick a pencil into the half-glass of water and observe. You will see what appears to be a broken pencil, the part above water not exactly connecting with the part underneath the water. That is what refraction is, a bending of the light waves due to the speed of the waves being slowed down. This "slow down" is caused by the material, or medium, the wave is having to travel through. More dense media cause the wave to slow, producing a different visual appearance.
Water is optically denser than air, that is to say that the speed of light in water is less than the speed of light in air. When light travels from an optically rarer medium to an optically denser medium, it bends away from the normal( perpendicular) drawn at the point of incidence such that the angle of incidence is greater than the angle of refraction.
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