Have you ever wondered what the world would look like if our eyes could see infrared and ultraviolet light? What if the visual spectrum of light was wider? What if we could see the entire electromagnetic spectrum?
Currently, we know that blue plus yellow equals green, but would that change if our eyes could detect more wavelengths of color? Technically- no, but that doesn’t mean that the world we see would stay the same!
Humans are trichromats, meaning we have cones sensitive to red, green, and blue light. Some animals, such as the zebrafish, actually have an additional cone in their eyes that sense ultraviolet light. These animals use this additional pigment for things like foraging for food or selecting a mate. We can deduce what humans would see with this frequency enabled from things like photography using certain lenses, but also from actual experiences. Later in his life, Claude Monet had cataracts surgery which entirely removed the lens of his left eye. The human eye lens actually prevents ultraviolet light from penetrating. Aphakic people, or those who have had a lens removed, have reported to be able to see some frequencies of ultraviolet light. This light is perceived as a whitish-blue or whitish-violet. Flowers would change colors drastically and many would actually have white tips. ExtremeTech has a great writeup about ultraviolet light and even how to see some “impossible colors”.
On the other end of the spectrum is infrared light, which is an interesting region since our bodies actually emit light of this frequency. If our eyes could see infrared light, we would actually glow! Additionally, we would be able to see each other through things like clothes and some walls. Our bodies aren’t the only things that emit IR light. Any heat source would glow with our infrared-enabled eyes.
If you could see x-rays, you would no longer be able to sleep without an iron eyemask since x-rays can so easily penetrate our skin, so you would not be able to turn off your x-ray vision. However, blinking would be quite an interesting experience since whenever your eyes were closed you would no longer see the current visible spectrum, but would still see x-rays! However, since x-rays are not as common as other regions, it wouldn’t effect our vision as much as the lower frequencies. The atmosphere contains a very small amount of x-rays, so the sky would have a slightly altered color.
Our sky actually glows with microwaves. If our eyes could see microwaves, the universe would look entirely different. @fadesingh actually simulated what the night sky would look like if we could see microwave radiation (in addition to visible light). The background radiation is the afterglow of the Big Bang and known as CMB, or Cosmic Microwave Background. The CMB also has a uniform brightness.
Since gamma rays are not as common as other frequencies in the spectrum, our eyes would not see them from many sources. Gamma rays are produced by things like radioactive atoms and in astronomical phenomenon such as supernovae. There are very minute traces of gamma rays in nature such as Potassium-40, so our vision would be slightly effected, but not as much as by other regions of the electromagnetic spectrum. Large doses of gamma rays would kill you, so you wouldn’t be able to see them in high amounts anyway.
Radio waves are the lowest range of the electromagnetic spectrum, and since everything in the universe emits radio waves, seeing them would be ultimately useless. Our retinas would be bombarded by these waves and would oversaturate our brains with a color far beyond our current red. However, you would not have much time to appreciate this new color because you would be blinded. This Quora post is a great read and explains how our eyes would be able to see over 7,000,000 shades of colors (instead of our current 300)!
May 24, 2013 Update: Looks like a human tetrachromat was confirmed. Click here to read more.