You're a lonely little [[photon]], traveling through space. What color are you?
(css: "background-color:red")[(colour:"red")[....]] [[Red]] (css: "background-color:red")[(colour:"red")[....]]
(css: "background-color:green")[(colour:"green")[....]] [[Green]] (css: "background-color:green")[(colour:"green")[....]]
(css: "background-color:blue")[(colour:"blue")[....]] [[Blue]] (css: "background-color:blue")[(colour:"blue")[....]]
[[Humans can't see me!]]
(css: "float:right")[ [[Credits]] ]A photon is a single unit of light, or [[electromagnetic radiation|EM]]. This can be represented as either a particle or a wave, since both can be used to model it.
A photon can be identified by its [[wavelength or frequency|wavelength]].
All photons have the same speed of 3 x 10<sup>8</sup>m/s<sup>2</sup>. This is known as the "cosmic speed limit" since no other particles have been found to move faster than this.
Photons don't have mass.
(link-undo:"Back")(css: "background-color:red")[Red light] has a [[wavelength]] of 650 nanometers. At sunrise and sunset, the light you see has traveled a longer distance through the atmosphere since the Sun appears closer to the horizon. The atmosphere scatters more of the high-energy (blue and violet) light, so colors like red and orange are easier to see.
$TryNewOrLeave(css: "background-color:green")[Green light] has a [[wavelength]] of 510 nanometers. The sensitivity of the human eye peaks at this wavelength. This is due to years of evolution - if we had evolved on a planet circling another star, the human eye would be more sensitive to a different wavelength of light.
$TryNewOrLeave(css: "background-color:blue")[Blue light] has a [[wavelength]] of 475 nanometers. This wavelength is most easily scattered by Earth's atmosphere, which is why the sky is blue.
$TryNewOrLeaveWell, even if you're not visible, you're still a valued part of the [[electromagnetic spectrum|EM]].
What might humans know you for?
[[X-Rays]]Radio waves have the longest [[wavelength]] of any light. They have wavelengths that can be measured in meters or even kilometers!
$TryNewOrLeaveHumans can't see infrared light, but we can feel it. The energy of the light in the infrared part of the spectrum is why lightbulbs and fire get hot and why you can feel the warmth from the Sun on a nice day.
The [[wavelength]] of infrared light is slightly longer than that of visible light.
$TryNewOrLeaveLight in the Ultraviolet part of the spectrum is harmful to humans' skin because of its high energy, but luckily the Earth's atmosphere blocks most of it. The [[wavelength]] is shorter than visible light, which is what contributes to its higher energy.
$TryNewOrLeaveX-Rays and the even shorter [[wavelength]] Gamma Rays have so much energy that they are very harmful to humans. However, x-rays are very useful to help diagnose patients, so they can be used - but very sparingly. Gamma rays are released in nuclear explosions.
$TryNewOrLeaveWavelength vs Frequency
<img src="https://www.savvius.com/images/compendium/FDSignl.gif" height="250" alt="Lower frequency is a longer wavelength">
(link-undo:"Back")(set: $EM to "<img src='https://upload.wikimedia.org/wikipedia/commons/3/3a/BW_EM_spectrum.png' height='300' alt='EM spectrum image'>")(set: $TryNewOrLeave to "\n[[Try a different photon!|Start]] \n[[Exit]]")(set: $color to "<img src='http://photomachining.com/images/mm/laser-wavelength-chart.jpg' height='300' alt='EM Spectrum'>")(set: $hf to "Who first discovered you?")I hope you enjoyed being a photon!
Here's the entire [[Electromagnetic Spectrum|EM]]:
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(link-undo:"Back")Double-click this passage to edit it.Infrared waves were discovered in 1800 when Sir Frederick William Herschel split a beam of light using a prism, and noticed that there was an increase in the temperature of the area the light touched compared to the temperature of a dark area. However, there was also an even higher increase in the temperature of the area just beyond the color red. He realized that this meant there was more light there that he couldn't see, but could still measure.
(link-undo:"Back")Double-click this passage to edit it.Double-click this passage to edit it.The electromagnetic spectrum is the range/spectrum of every wavelength of light. The scientific name for light is electromagnetic radiation.