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I'm bit confused about 'black' as a color. As per my knowledge, it is not given in visible color spectra like other colors for example red, violet etc. Also I'm confused with definition of color--does it mean the reflection of respective wavelengths from an object perceived by human eyes or latest electronic sensors determines the color of that object? This is okay for normal people to know that this this color is this this. But how will you tell that some object has Red color to a blind person, how that person can think of that? I can define a bicycle as a vehicle with two wheels, a handle, a seat--this way, a blind person can imagine that vehicle, but tell me the definition of color so as blind person can imagine about specific color considering he/she is blind since birth. Also one more question regarding black--what is the color of an apple if it is placed in a dark room--to eyes, it seems blindly blackish (oh, what the hell this black is again, and why we feel blind in dark places although eyes are open, huh?), but if we focus a torch on that apple with a daylight like color, it looks with original color, why so?

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Related: physics.stackexchange.com/q/16691/2451 –  Qmechanic Mar 3 '12 at 9:12
    
Also physics.stackexchange.com/q/21336/124 –  David Z Mar 3 '12 at 19:41
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5 Answers

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The human eye has three types of colour receptors, called cones, that respond to red, green and blue light. Your brain interprets the colour based on the signals from these cones.

For example suppose you're looking at red light. Only the "red" cones will generate a signal and your brain interprets this as red. Suppose now you're looking at a mixture of red and green light. This time the red and green cones generate signals while the blue cones do not, and your brain interprets this as yellow. If there is no light entering the eye none of the cones generate a signal and your eye interprets this as black.

Some colours are pure, that is they consist of light with only a single wavelength, but most colours are mixtures of light with lots of different wavelengths. There are lots of ways scientists measure colour e.g. the RGB system, but actually this turns out to be a surprisingly complicated thing to do because many measurement schemes can't measure all possible colours. Have a look at http://en.wikipedia.org/wiki/Colour for more info.

Finally, to take you example of a red apple: the apple is red because it reflects red light but absorbs other colours. If I shine white light from a torch onto the apple only the red light is reflected and enters my eye, so I see the apple as red. Suppose I shine pure blue light (wavelength about 450nm) onto the apple, what will I see. Well the apple will absorb the blue light so no light enters my eye, and the apple looks black.

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Hmm, it's thorough. Thank you @John. –  Vishal Mar 27 '12 at 15:32
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Dear Vishal just read this http://en.wikipedia.org/wiki/Blindness

A 'red' apple does not emit red light. Rather, it simply absorbs all the frequencies of visible light shining on it except for a group of frequencies that is perceived as red, which are reflected.

This is wrong an apple looks red to us because it reflects infrared waves..
hope you understand that why an apple appears to be red . now your next question that -what is the color of an apple if it is placed in a dark room--to eyes, it seems blindly blackish...The reason is still the same..Actually black light does not contain that combination of frequencies which an apple reflects ...It absorbs all the black light falling on it and hence we perceive it as black.....MOST IMPORTANTLY EVEN IF WE KEEP THAT APPLE IN A ROOM CONTAINING FREQUENCIES CORRESPONDING TO GREEN LIGHT THEN ALSO IT APPEARS TO BE BLACK AS IT ABSORBS THAT GEREEN LIGHT FALLING ON IT...And you should once read about phenomenon of seeing the objects in the world around us and then you will be able to understand it..

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Yes, I know that it appears black when all the frequencies are get absorbed and it reflects/emits nothing -- but I want to know about what this black (dark) is, how to define it (let us consider that I'm a blind person) and same as with color. –  Vishal Mar 3 '12 at 9:13
    
john rennie also gave the same explaination as mine... –  Manisha Mar 3 '12 at 9:38
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tell me the definition of color so as blind person can imagine about specific color considering he/she is blind since birth

Colour perception is purely subjective. I can't even guarantee that you perceive red in the same way I do; at best we can only agree they're the same. As a result, you cannot explain colour to a blind person. The best you can do is to compare it with other things. For example, explain that red differs from green in the same way that a low pitch note differs from a high pitch. That does not explain how we see an apple as red, it just tries to explain that different colours have different degrees of "something", just as pitches have different sound frequencies.

To see the difficulty, try to explain how the signals from 2 radio stations differ. We can't perceive them directly, and can only explain by comparing them to something like light: "one station is redder than the other" or sound: "this station broadcasts at a lower pitch".

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Good answer @hdhondt! However, there are quite measures available for recognizing the intensity of the pitch of sound frequencies; are same techniques are applied for measuring strength (or intensity) of darkness or low-presence of a color (or say light)... So when it is the dim light at evening, it should be understandable by just measuring its intensity. I think my question is not representing what I want to know. :P –  Vishal Mar 27 '12 at 15:25
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You see. At first there is nothing. Nada, null, zero, nothing. This is dark. Perceived as black.

If you add light you will get the absence of darkness. Light is photons, electromagnetic radiation, that is picked up by the rods and cones in the human eye. Photons oscillate as they travel through space. The difference in the wavelength as they travel gives the very difference in colour. The spectrum of visible light is from violet at about 380nm to red at about 720nm. The rest is invisible light. Defined as either ultra violet or infra red rays.

You cannot tell me what ten is, without having defined zero.

The same way I mean you cannot tell me what red is without having defined no colour. So if you ask me, black is the colour of zero colour.

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Black is not really a colour. It is an absence of colour, or more generally, of light.

The perception of colour is due to some light entering your eyes. If no light enters your eye from some particular place, that place will appear black. This explains why a dark room appears black: because there is no light entering your eye from any direction.

If an object is black it is because it is not emitting (or reflecting) any light in your direction. It might be more accurate to describe something as dark, rather than black.

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Sure, got it! Black means absence of color--sounds something similar like void in programming! ;) –  Vishal Mar 27 '12 at 15:29
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