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A question about rainbows

  1. Aug 9, 2004 #1
    I know that water drops act as a prism to turn white light into the visible spectrum.

    What I don't understand is how come many prisms are meant to seperate the colours. What I mean is the following.

    One prism would make red, orange, yellow, green, blue.

    Two prisms would not make red red, orange orange, yellow yellow, green green, blue blue. It would make red, orange, yellow, green, blue, red, orange, yellow, green, blue.

    1 prism: http://homepage.ntlworld.com/ak_gara/Rainbow_1.gif

    2 prisms: http://homepage.ntlworld.com/ak_gara/Rainbow_2.gif

    Many, many prisms: http://homepage.ntlworld.com/ak_gara/Rainbow_3.gif

    How is the red from all over meant to go to the left, and all the blue from all over go to the right, and all the green all over go to the middle?
    Last edited by a moderator: Apr 21, 2017
  2. jcsd
  3. Aug 9, 2004 #2
    In actuallity, you should think in terms of humidity as a function of height (or rainbow radius). The entire atmoshere in the region is the "droplet". The lesser the altitude, the greater the humidity density, so that the atmosphere's index of refraction is graded. Since the angle of refraction is function of wavelength, white is divided in the colours. Note that red is always on top in a rainbow, while purple, the shorter wavelength is the most refracted.
  4. Aug 9, 2004 #3

    Claude Bile

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    The reason that the colours appear spread out, as in a rainbow effect is due to dispersion (variation of refractive index with wavelength). Red appears first because it has the longer wavelength, blue last because it has a shorter wavelength. There are also invisible infrared and ultraviolet wavelengths present, but we can't see them, of course.

    A rainbow appears under the right conditions because the atmosphere forms a crude diffraction grating. If you look carefully next time you see a rainbow, you may be able to make out a second rainbow above it, that is considerably dimmer and blurred compared to the other rainbow.

    The first rainbow (the bright) one is often termed the 'first order' diffraction maximum, while the second, dimmer one is termed the 'second order' diffraction maximum.

  5. Aug 9, 2004 #4


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    You envisioned your prisms sitting on top of each other, each reflecting a separate beam of light. One wide beam of light going through a bunch of prisms, all oriented the same way, results in all red light being refracted a certain amount, all orange a certain amount, and so on - in other words, one big rainbow.

    Since the Sun is bigger than the Earth, the light comes in parallel - one big beam of light. All water droplets refract light. Only the water droplets that are a certain angle relative to you and the Sun result in you seeing a rainbow. In other words, two people standing across the street from each other will see the 'same' rainbow, but not through the exact water droplets (so, I guess technically, you could say that each is looking at their own rainbow and both rainbows are just identical).

    The necessary angle to produce a rainbow includes an entire circle of water droplets. Except, when you're standing on the ground, the bottom half of the rainbow is wiped out by the ground. If you looked at a rainbow from an airplane in the sky, it would be a circle.
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