Light - Physics of Light
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Light

Rainbow
Rainbow over Amazon river

Light is made of billions of tiny particles called photons. These photons travel from one place to another in waves. Visible light is the subset of photons that move at a wavelength that we can see. Among the different photons that are in visible light, the ones that have the longest wavelength look red to us, and the ones that have the shortest wavelength look blue to us. All colors come from different wavelengths of light.

Most of the light on Earth comes to us from the Sun. The Sun shoots out billions of photons every second in all directions, and the ones that happen to be pointed toward the Earth come here. When these photons get to the Earth, they first run into Earth's atmosphere. Some of the photons get absorbed by the atmosphere itself, especially the ultraviolet ones whose wavelength is shorter.

Blue Sky
Blue sky over Tunisia's olive orchards

This is a good thing for us, because too much ultraviolet light would kill us, and all other living things on the planet. (In fact, doctors sometimes use ultraviolet light to kill germs!). Because blue light has the shortest wavelength of the visible light photons, some of the blue light is also absorbed by the atmosphere. That's why the sky looks blue from the ground in the daytime, because you can see all that blue light up there. But when the light hits clouds instead, the clouds reflect all of the light down to Earth, so the clouds look white to us.

Most of the visible light does get through the atmosphere and comes down to the surface of the Earth.

In addition, a lot of infrared light also gets through the atmosphere. This is light that has a longer wavelength, so we can't see it.

Learn by Doing - using glowsticks to see how colored light mixes to make white light.

Bibliography and further reading about light:

Electricity
Atoms
Physics
Chemistry
Biology
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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