When light enters an optically denser medium the light ray

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I am a first year undergrad student doing optometry (never done any physics before in my life :( ). I got a question asking what happens when light enters a denser medium. I was told that the frequency of light doesn't change, but the wavelength decreases. Is that true? And if so, why does the frequency never change? As little maths as possible would be great - this subject is mostly very qualitative and I'm not a maths expert.

When light passes from a less dense to a more dense substance, (for example passing from air into water), the light is refracted (or bent) towards the normal.

The normal is a line perpendicular (forming a 90 degree angle) to the boundary between the two substances. The bending occurs because light travels more slowly in a denser medium.

A demonstration of refraction can be conducted at home in a dark room. All that is needed is a flashlight, a clear glass filled with water and a small mirror.

• Figure (a): Shine the light directly into the glass. If the light strikes the water straight on (or parallel to the normal), no bending occurs and it simply passes directly into the water undisturbed, leaving only a straight beam of light all the way to the bottom of the glass.

• Figure (b): Shine the light into the glass at an angle. As the light enters the water, it is refracted. Since the light is passing from air (less dense) into water (more dense), it is bent towards the normal. The beam of light would appear to bend at the surface of the water.

• Figure (c): Place a mirror at the bottom of the glass of water and again shine the light into the glass of water at an angle. As light initially enters the water, it is refracted as in figure (b) and then reflected off the mirror (at the bottom of the glass). Upon exiting the water, the light is bent away from the normal as it passes from water (more dense) and into air (less dense). The light would leave the flashlight, bend at the surface of the water, reflect off the mirror at the bottom of the glass and move towards the surface, where it would bend outward at the same angle it bent in on the way in.