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Physics for all



Myth: Owls and other animals can see ultra-violet or infra-red light. If an object was coloured infra-red or ultra-violet, how would this hypothetical object appear to humans?

The difference between the colours is the wavelength: red colours have longer wavelength - red colour has wavelength of around 700 nano-meter, while the wavelength of blue colour is ~450 nano-meters. The human eye has a limited wavelength sensitivity: most people can see only at wavelengths between 380 - 750 nano-meters, namely between red and blue. Light at wavelength shorter than 380 nano-meter is called "ultra-violet", and light at wavelength higher than 750 nano-meter is called "infra-red". We cannot see such wavelengths; thus, if an object would have such a colour, it would appear dark (black) to us.

However, some animal's eyes are sensitive to wavelengths that we cannot see. Bees and other insects, for example, (as well as many birds) can see ultra-violet light; some plants indeed have ultra-violet colours in their flowers!

All objects that are hotter than absolute zero emit radiation. Objects which are at temperatures of ~30-40 degrees Celsius, emit at the infra-red. These include many animals, including humans (our normal body temperature is 37 degrees Celsius). Thus, in fact all people emit radiation in the infra-red. Our eyes are not sensitive to that; thus, if you enter a dark room and there is a person in it (or, say a cat), you couldn't see this person (or the cat). Some animals can see in the infra-red: the most notable ones are snakes and some types of beetles, who use this skills to detect forest fires (and lay their eggs there).

Night visions equipment - for security (such as border control) and military uses often makes use of the fact that humans emit radiation in the infra-red, to detect the existence of humans. These work even in the darkest night, where the normal human eye sees nothing.

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