Question

What is the thing in the prism that splits it into seven colours?

Answer 1

Whenever a photon encounter a different media, for example from air to glass, the electromagnetic wave will slow down in speed inside the glass, since glass is denser than air. If the photon or wave came in an angle, the wave side that first hit the glass will slow down first, what will “bend” the whole wave in that direction. The whole wave will then travel in this new direction until it hits another change direction condition. A prism has no parallel surfaces, so the wave will again change direction since now leaving glass to air will speed up the wave again, but to the direction where the air comes first (the wave is not perpendicular to the glass surface). As the surfaces (entry and exit) are not parallel, the wave will not resume the same entrance direction, and will travel into the air in a slight different one. Now, depending on the wave (photon) frequency, this change in direction from air to glass to air, will be more or less pronounced. Higher frequencies (blue side of the spectrum) will have more steep angle, change in direction. Lower frequencies (red) will have less steep angle. This is why a “rainbow” forms at the output of a prism. Now, a glass slab having both surfaces (entry and exit) parallel, they will change the direction of the waves at entrance as the prism does, but they will recombine the directions at exiting the glass to exactly (almost) the same direction as it entered. This is why you don’t have a rainbow through a glass slab.