Why do we see colour Visible light consists of electromagnetic radiations of wavelengths 400 – 750 nm. Each wavelength is associated with definite energy and produces characteristic colour sensation when it strikes the retina of our eye. Light possessing all the wavelengths from all the wavelengths from 400 – 750 nm appears white. The white light is actually made of seven colours. These are violet, indigo, blue, green yellow orange and red (VIBGYOR). When white light falls on a substance, three things may occur.
i)             Totally reflected, i.e., nothing is absorbed.
ii)           Partially reflected and partially absorbed
iii)          Totally absorbed, i.e., nothing is reflected.
The substance appears white when light is totally reflected or black when it is totally absorbed. But when a part of light is reflected and rest absorbed, the substance looks coloured. If only a single band is absorbed, the substance will have the complementary colour of the absorbed band.
In case only one band is reflected and rests are absorbed, the colour of the substance is that of the reflected band. Thus, the substance may appear red,
(a)Either because it absorbs only the green blue band (490-500 nm) from the white light and reflects all the rest.
(b) Or because it absorbs all the bands except red (605-750nm), which it reflects.