Transcript:

So here's the thing about waves. Whenever a wave changes its speed, when something causes a wave to change its speed, it also changes its direction. This is important. Why?

Because this is what explains why, in a prism, we can take white light, and it splits into the visible spectrum — violet, indigo, blue, green, yellow, orange, and red. At one end of the prism, when the light goes from air to glass, it changes speed, and each of the wavelengths turns or changes direction by a slightly different amount because it is a function of the wavelength.

At the other side, when it exits from glass to air, it again changes its direction, and it splits. There is a further split, so the white light is resolved into the seven visible colours — violet, indigo, blue, green, yellow, orange, red.

This is also the reason why, if you notice on a beach, waves always come parallel to the beach. They never come at an angle. It is precisely because of this — because the wave changes direction based on the depth of the ocean, and as a result, it comes in parallel.