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Name: Tiffany Y.
Status: student
Age: 8
Location: N/A
Country: N/A
Date: 4/22/2003


Question:
Can you see a rainbow when the sun is blocked by clouds? If yes, how?


Replies:
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 dispersion.

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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