|MadSci Network: General Biology|
This is a very interesting question! First let’s bring the whole class up to date with a brief (because I’m a biologist and not, in fact, an optical engineer) description of color. When white light – such as sunlight or light from a halogen lamp – hits a surface, that surface can often absorb certain wavelengths of that light. Recall that the typical spectrum of visible light includes seven colors: Red, Orange, Yellow, Green, Blue, Indigo, and Violet. If the surface absorbs all but the blue wavelengths of light, then the human eye will perceive that object as being blue, and if it absorbs all but red, then it will be perceived as being red. If it absorbs none of the visible light, then we will perceive it as white, and if it absorbs all of the colors we will perceive it as being black. Simple enough, eh? But there’s a catch. Just because humans can only perceive those seven colors of light doesn’t mean that the energy that light is comprised of can’t be higher or lower in wavelength…wavelength is the property of this energy (electromagnet ic energy) that makes it one color or another. If electromagnetic waves have a shorter wavelength than the shortest light wavelength, violet, that energy can be ultraviolet light (like a black light), X-Rays (like at the doctor’s office), or potent radiation called gamma rays. If electromagnetic energy has a wavelength longer than the longest light wavelength, red, it can be infrared energy (like a heat map), or radio waves (like AM/FM).
I would also like to quickly introduce the concept of refraction, or how light behaves when passing through an object. If you saw a drop of clear oil in a bowl of clear water, you could still see it, right? That’s because the oil has different refractive properties than water, and we can see the different when they are side by side. The water bends the light one way, and the oil bends it a different way.
Now, back to wavelength and EM waves.
Some animals can perceive electromagnetic energy beyond the visible light spectrum we see. Insects, for example, are excellent at seeing things in ultraviolet. Some plants, for example, have appearances in the ultraviolet spectrum that makes them especially alluring for bees.
Okay, let’s get to the question at hand. You ask if something is another color other than visible light. Well, any opaque object has to have a color. If visible light is reflected off of something or absorbed by something, our eyes will detect that event and assign it a color. It is certainly possible that the object could have other properties in other EM spectra, like the flowers I mentioned above, it will still have properties in the visible light spectrum as well.
One intriguing possibility, though, is what if an alien were not opaque, but were transparent. Transparent objects are able to have no color because instead of reflecting or absorbing visible light, they allow it pass right through. That’s why glass can be perfectly clear and colorless. There are certainly examples in nature of animals that are almost transparent. Jellyfish can be very difficult to detect in water because they lack opaque organs. The same goes with some small aquatic insects…this provides an advantage to these organisms because they are more difficult to detect by predators. If any alien were completely clear, and the tissues of their body affected light in a similar way to the how the air around them did (if the oil in the above example had the same refractive quality of water), then it would be exceedingly difficult to detect them visually. And so the possibility you raise is possibility, albeit remote.
I hope this helps you, and keep wondering!
This is a nice site that demonstrates how refraction works:
NASA’s page on EM energy (explained much more eloquently than I can):
This is a neat site that talks about so-called “cloaking” technology. It’s interesting but I don’t vouch for its content:
A nice introduction to color:
Wikipedia has some good articles of EM energy and light properties. Remember that Wikipedia isn’t always a reliable source and should probably only be used for background purposes:
Try the links in the MadSci Library for more information on General Biology.