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Why different objects have different colour? When a white light (composed of all colour) falls on a red book, is it true that only red colour is reflected back? If no, then why it appears red. If yes, then why does the book doesn't get warmer even if it absorbed rest other wavelengths. How is hot red iron rod different than red book?

Kindly help in clearing this confusion.

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The wikipedia article on color can clear the confusion on color perception. The electromagnetic spectrum in the visible range corresponds to colors as the human eye observes them, but color perception is more general and depends on the phsyiology of the eye and brain. One can make color photographs using for illumination only two frequencies, for example, as polaroid inventor E.Land demonstrated.

A hot iron rod is red because the upper end of the infrared spectrum, which is the heat you are feeling/measuring , not seeing, goes into our perception of red. The overwhelming majority of the energy is in the infrared for which we have no retina sensors, only skin ones.

A book may be red because it is reflecting the red frequency, or we may perceive it as red because of the cones in our retina ( read the article of color in wiki). The cover absorbs the non reflected part of the spectrum but that is a very small amount of energy to be converted to heat from room light, so we do not perceive the difference in temperature.

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Another view may be; when Sun light strikes the white roof of a hot rod. Even in the blazing summer heat it will be cool to the touch. Evidently NO light is absorbed because the metal is cool. Any other color is to hot to touch but for only a moment!

A more colorful view of reflection is Sun light striking dew or rain drop at the ends of the leaves on Wax Privet bush. The many layered colors of the Sun are reflected in these drops but only in a small bandwidth ie; red goes to violet, green may go to yellow. And this is only visible if the wind moves the branch, or the observer moves the light of sight.

When the white light from the night light strikes these same dew drops it is only reflected in one the one color. By this observation I would say man-made light is produced only in one part of the color spectrum, which means that yes, the red book will always remain red to the observer because the light origin is only of one part of the spectrum.

The red book will remain red in Sun light because it is colored red and the Sun light is white or non-reflecting. Maybe.

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"Evidently NO light is absorbed because the metal is cool." Generally not true; little light is absorbed and so there is little heating, but this is not a place to use an absolute. –  dmckee Apr 8 '12 at 17:55
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