If light hits a window it bends and comes out parallel, but if it hits a prism it breaks into a rainbow. How does it know the shape of the glass it is entering?
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$\begingroup$ Does this help? en.m.wikipedia.org/w/… $\endgroup$– my2ctsCommented Jun 15 at 7:20
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As you might know, light is made up of different colours of light. Each colour has a different angle when it transitions from one medium to another.
For a window, since the surface of the window that light is entering through and the surface of the window that the light is leaving through is parallel, even though the angle of deviation is different for different colours, once the light leaves the medium, they have the same angle that they entered with, as you can see in the diagram here:
For a prism, since the surface of the prism that light is entering through and the surface of the prism that light is leaving through is not parallel, the different colours of light leave with different angles of deviation, forming a rainbow that you can observe, as you can see in the diagram here:
The key difference is, that in the first case, for each light, the angle of refraction at the first surface = the angle of incidence at the second surface, so the angle of incidence at the first surface = the angle of refraction at the second surface, causing the light to still appear white.
But for the second case, angle of refraction at the first surface ≠ angle of incidence at the second surface, so angle of incidence at the first surface ≠ angle of refraction at the second surface, and the light diffracts and you can see the rainbow.
