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Desert Diary
Physics/Bent Light


Most of us remember the speed of light as around 186,000 miles per second. We sometimes forget, though, that this is the speed of light in a vacuum, and it's quite different when traveling through other substances. That makes for some interesting phenomena, such as a stick stuck into water seeming to bend at the water's surface. This is because light travels slower through water than through air, and so is bent downward as it enters the water.

We can even thank the phenomenon for a few extra minutes of light every day. As the sun's rays move from the near vacuum of space into the atmosphere, they are bent. At sunset, light that otherwise would pass above our heads is bent downward—we see the sun for moments after it has really sunk below the horizon.

Light travels faster in hot air than in cool and thus our desert mirages as light from the sky is bent upwards, entering our eyes as if it originated from the ground. The seeming pool of water really is a pool of sky.
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Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.