visible-light
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.
The amount of reflected light is predicted by Fresnel's equations. The equations are a little cumbersome, but they tell you that the fraction of light reflected depends on the index of refraction of the two materials, as well as the angle of incidence.
One thing though, is since the index of refraction changes with the wavelength of light, then the amount of reflected light also changes with the wavelength!
Realistically though, the index of refraction barely changes at all over the visible spectrum.
In this image, the orange band is the visible spectrum. Those materials labeled "crown" are different types of glass. The range of the index of refraction for these glasses is only about 0.02 or 0.03 over the visible spectrum.
In other words, the glass barely notices the different colours. It reflects all visible light almost identically.
Your "more precise phrasing" is indeed exactly right.
Best Answer
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.