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Rainbows from Right Angle Prisms (sans SVG¹)

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.

White light in on left leg, rainbow exits through hypotenuse on the right.
Fig. 1.  White light enters the prism from the left; a rainbow is on the right.

White light in through hypotenuse, rainbow exits through leg on the right.
Fig. 2. Reversing directions, light enters along the hypotenuse, exiting as a rainbow.

White light enters through second leg from bottom, rainbow exits through hypotenuse at top. White light enters through hypotenuse from the bottom left, rainbow exits through triangle leg at top.
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) 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.

White light ray enters near to square angle between the legs of the triangle, reflects off the second leg, and exits through the hypotenuse forming a rainbow.
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.

¹ SVG is the acronym for Scalable Vector Graphics. The SVG version of this page is at Right Angle Prism Rainbows.

Get a right angle prism.

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