A Staff Report from the Straight Dope Science Advisory Board

Why is visible light visible, but not other parts of the spectrum?

March 27, 2003

Dear Straight Dope:

As the sun ages and changes the light it emits (from red giant to blue/white dwarf, etc.), will it affect how color is perceived by life on earth (if there is any left by then)? Is today's "yellow" sun directly responsible for the array of colors we perceive? --Case Cooper

Why is the "visible" portion of the electromagnetic spectrum where it is? Why isn't it slightly up or down the wavelength slide? Why can't we see (or otherwise sense) heat, UV rays, radio or other electromagnetic phenomenon? Any light you can shed would be beneficial to my piece of mind.

Shed some light? Listen, pal, making dorky puns is a union job here at the Straight Dope. (The position is currently filled bySDSTAFF Dex.)

Our sun is directly responsible for the electromagnetic band we perceive as "visible." The sun emits a characteristic electromagnetic radiation spectrum according to the famous Planck Black Body Radiation law. The shape and peak of the EM intensity as a function of wavelength is dependent on the temperature of the emission source, in this case the sun at 5780 degrees Kelvin. The intensity peak of the EM radiation that the sun emits is in the visible wavelength band (wavelengths from 400 nm to 700 nm). Evolution takes care of the rest. If your light-sensing organ is sensitive to some band of radiation that is, say, only half as intense as the visible band, then come dusk, you are pretty much dinner for the life forms that sense the intense visible band.

If the sun were to start emitting its intensest radiation in the X-ray band starting tomorrow, your light sensor organs would still be tuned to the visible band--you'd still see wavelengths of 550 nm as "green." Whether human eyeballs would evolve to match the sun's peak intensity as the peak shifts I can't say, but you can read what The Master has to say about continuing evolution. Of course, we don't expect the sun's spectrum to change much for the next 10 billion years. By that time, if humans are still around, we'll probably have invented some fancy glasses or something to help us to see in any EM spectrum range.

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