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Asaf Pe'er

Physics for all



How can you make black if black is the absence of colour?

When you shine light on material, three things can happen. First, part of the light will be absorbed. Second, part of the light may be reflected. If the material is transparent, then a third option is that part of the light will be transmitted through the material (for example, glass). Note that, apart from very few exceptions (like the sun, fire, light bulbs or computer screens), most of what we see is really light that is reflected from objects; thus, in the absence of external light - e.g., in the night or when there is no electricity, we don't see much, and everything appears dark.

What our brain interprets as dark, or black, is the lack of photons that get to our eyes. Thus, the way to make black is simple: we would consider a body to be black, if all the light that we shine on it is absorbed by the material, while nothing is reflected back to us. If, on the other hand, a material reflects back light at all wavelengths, we call it white. A good example is snow, which is an excellent reflector - it does not shine its own light, but it reflects very well light that shines on it. This is why people who ski often wear special goggles - to avoid being blinded by the reflected light.

If an objects reflects only one (or a few) wavelengths, while absorbing other wavelengths, we say that this material has color: for example, if material absorbs most of the light that shines on it, but reflects light at wavelength of 620 - 750 nano-meter, we would normally refer to such object as "red". If it reflects light at 450-500 nano-meter, it will appear blue to us. Since the human eye is sensitive only in the range (roughly) 380 - 750 nano-meter, any material that absorbs all the light in this range will appear dark (or black) to us, even if it reflects light at other wavelengths. Indeed, other animal's eyes are sensitive at other wavelengths than our own eyes. Bees for example can see ultra-violet light at 300 nano-meter. For humans, this will look dark.

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