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Refraction, Dispersion, & the Right Angle Prism

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Rainbows from Right Angle Prisms

Contrary to what you might have seen in places on the Internet, you can make rainbows using a right angle prism. The basic ways to get a rainbow are shown in the diagrams. (No images? Go to Right Angle Prism Rainbows sans SVG.)

Fig. 1.  White light enters the prism from the left; a rainbow is on the right.

Fig. 2. Reversing directions, light enters along the hypotenuse, exiting as a rainbow.

Fig 3.  (a) Symmetry arguments lead us to input light along the second leg. White light colors are separated into a rainbow above the prism on the right.  (b) Finally, light enters the hypotenuse, and exits creating a rainbow.

If you're careful, you can get a rainbow after an internal reflection. The light ray must enter near to the square point between the two legs of the triangle. The result is shown in Fig. 4.

Fig. 4. A rainbow is formed after an internal reflection.

Great rainbows begin with sunlight. Take your right angle prism outside and try it. If you have trouble finding the rainbows, bring a friend along to help you look for them.


Get a right angle prism.


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