What is it that determines the colour of anything?
MOHAMMED ABDUL SHAKOOR
Colour is the visual perception of the physical signal, the light, that eye receives. The colour depends on the kind of light that falls on anything and also that bounces off or passes through it. Light travels in waveform and each colour has its own wavelength. So, in other terms, colour depends on the wavelengths of light that reaches the surface of an object and that reach eyes after being reflected or transmitted by the object.
Surface of any object has, in general, three effects on light falling over it — absorption, reflection and transmission. If an object absorbs all wavelengths of light falling on it, then it appears ‘black’ and if it reflects every wavelength of light falling on, it will appear ‘white’.
If an object reflects wavelengths of light partly, absorbing the rest, then the reflected wavelengths ‘decide’ the colour of it. Thus the apparent colour of an object depends on the wavelength of the light that it reflects. For example, a ‘red object’ observed in daylight appears red because it reflects only the waves producing red light.
We see green leaves as being green because chlorophyll in the leaves, because of its nature and chemical makeup, absorbs the all other wavelengths (colours) of the sunlight (called white light as it contains all wavelengths), except green. The green is reflected back out to the viewer making the leaves appear green.
The light falling on the surface also determines the colour. If a piece of cloth which appears green in sunlight is viewed under red light it will appear black, because the pigments on the surface of that cloth have the property of reflecting only green light absorbing all other wavelengths falling on it. As red light has only wavelength corresponding to red colour it is completely absorbed and as no colour is reflected back it appears black.
Transparent objects do not reflect light, but transmit it. A ‘blue’ glass transmits only blue light, absorbing all other colours, so appears blue. A non-rigid ‘object’ like sky also has colour due to the phenomena called scattering. As sunlight strikes the upper atmosphere blue light is scattered the most by air molecules and suspended dust particles etc., which illuminates the whole sky, giving it its colour.
DR. N. PRITHIVIKUMARAN
Associate Professor in Physics
Virudhunagar, Tamil Nadu