Practice portal › Ray Optics and Optical Instruments › Refraction through a Prism
Asked in GSEB Board July 2022 · Dispersion and scattering
Given: the primary rainbow, made by sunlight in spherical raindrops.
Idea: follow one ray through a single drop.
It first crosses from air into water at the front of the drop -- refraction, and this is where the colours begin to separate, because water's refractive index is a little larger for violet than for red.
Inside the drop the ray reaches the back surface and is reflected there, once.
It then leaves through the front surface -- a second refraction, which widens the colour spread further.
So the order is refraction, reflection, refraction, and the light emerges about 42° away from the direction of the incoming sunlight, which is why the bow is an arc of that angular radius about the point opposite the sun.
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