Why does or doesn't a rainbow form?

  • #1
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Main Question or Discussion Point

When light enters some pieces of glass from the air, such as a magnifying glass or window, rainbows usually don't form. But when light enters a prism, rainbows form.

Why do rainbows form in the prism, but not in the magnifying glass or window?


(This is my own personal curiosity and because I tutor physics. I am NOT a student and do NOT have homework so please don't tag this post.)
 

Answers and Replies

  • #2
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You can split light into its components with a magnifying glass, but they are designed to reduce that effect.
Different colors are refracted to different angles - in a window, the effects of "air->glass" and "glass->air" cancel exactly because those surfaces are parallel to each other. This is different for prisms.
 
  • #3
Drakkith
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Why do rainbows form in the prism, but not in the magnifying glass or window?
A window's surfaces are parallel to each other, so the light that enters the window will leave at the same angle as it entered and you won't get a noticeable rainbow effect. A cheap magnifying glass can make a rainbow, but you may need to help it a bit: Take a piece of paper, cardboard, or other thin, opaque object and cut it into a circle that's just a bit smaller than the magnifying glass's lens. Now place it on the lens. You want only the outside edge of the lens to be visible. Shine a bright light source through it and you should see circular bands of colors around the focal point. The lens is acting like a round prism. (Note that this may be easily visible or barely visible depending on the quality of the magnifying glass, the power of the lens, and other factors.)

The paper blocks out all the light that would otherwise mix with and obscure the light. You want to block the center and shine light through the outer edges because the chromatic aberration is greatest near the edges of the lens than in the center.
 
  • #4
sophiecentaur
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To get a good spectrum display, you need everything to be just right. A 60oprism of good, lead glass does this (see this link) pretty well when you shine light from a vertical slit through it. Any old piece of glass of any shape will tend to split a beam of light into is constituent wavelengths a bit (even the most expensive camera lenses have a finite amount of 'Chromatic Aberration') You can think of a lens as a set of extremely small prisms at different angles and in different planes so that's not too surprising.It's all a matter of degree, how much the light is actually split.
 

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