Rainbows When Sun is Blocked
Name: Tiffany Y.
Can you see a rainbow when the sun is blocked by clouds? If yes, how?
It is possible for you to see a rainbow when your view of the sun is blocked by clouds, but the
raindrops that refract the light back into you eyes and so form the rainbow must be illuminated
by the sun.
White light entering a spherical raindrop is refracted backward at an angle between 140 degrees
from the incident light for violet light and 138 degrees for red light. Get a protractor and
draw a few examples, one with the sun near the horizon and one with the sun high in the sky.
Draw some clouds obstructing your view of the sun, but not your view of the raindrops or the
raindrops view of the sun. These angles differ (and so form a rainbow) because the index of
refraction depends on the wavelength; it is greater for
violet light than for red light. This dependence of the index on the wavelength is called
Notice that if you are on a level plain, you will not see a rainbow when the sun is more than
42 degrees above the horizon, since then all the light is refracted upward. If you are in the
mountains, however, you can sometimes see a rainbow formed by clouds down in the valley when the
sun is high in the sky.
If you play with a spray hose on a sunny day next summer, you can check these assertions.
Best, Dick Plano, Professor Physics emeritus, Rutgers University
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Update: June 2012