Almost All You Want to Know About Colour

Almost all you want to know about Colour

Rakesh Mohan Hallen

COLOUR is often a paradox. An apparently colourless light falling on a colourless water drop can give rise to a rainbow of colours, or one may see colours even when there is no object in view. What, then, is colour? Where does it exist, in the light or in the object? Colour is a sensory perception , caused by the interaction of light with our visual perception apparatus (eyes and brain). One may wonder, how the hues all around us originate. The answer lies in the study of the physical nature of light, its interaction with matter and the physiological as well as psychological understanding of vision.

Light is one of the various forms of ubiquitous energy. It can be produced from other forms of energy as it can be converted into them. Unlike matter it is not made up of electrons, protons or neutrons, rather it can be produced by their mutual interactions. Its behaviour can be explained, sometimes, by assuming that it is a kind of wave (which does not need a medium to travel through) and for some phenomenon by assuming that it is made up of particles having no mass (photons). When light interacts with matter it can be reflected back, absorbed or even transmitted through. It travels in a straight line, but it can change its direction when travelling through some kinds of matter. It has the attributes of quality as well as quantity. Which means that while it can be more or less it can also be of several kinds. One kind of light may be differentiated from another by the energy distribution of its photons (or distribution of wavelengths). A particular kind of light--called monochromatic- is made up of photons (or waves) having a single energy value (or wavelength). The nature (kind) of light often depends on its source. Light of one kind may be separated from another by its interaction with matter, and here lies the origin of various colours the eyes can perceive. As there are infinite kinds of light, so must be the number of colours. Then why do we often refer to VIBGYOR or seven basic colours of light. The origin of seven colour theory is credited to Sir Isaac Newton, who did the pioneering research in dispersion of light by a prism. He described the spectra to be made up of seven colours because he thought the colours must be in harmony like the notes of a music, and since there are seven notes in music, so there must be seven basic colours.

Now let us understand a bit of interaction of light with matter. Matter --as we all know--is made up of atoms and molecules which in turn are composed of electrons, protons and neutrons. The energy of electrons in atoms or molecules is quantized, which means that they can absorb or emit energy of discrete values and not any random value. So when light falls on matter--solid, liquid or gaseous -- the electrons of the atoms or molecules --which make up that matter --can absorb only light of a particular kind. The rest of the light may either be transmitted through or reflected back. There is another possibility of interaction of light with molecules-- this is called scattering. When molecules of a particular kind cannot absorb light of any particular kind, they can scatter the incident light. Scattering of light means that light of different wavelengths after interacting with the molecule start traveling in different directions.

How does the human eye perceive colours? Our eyes are sensitive only to light having wave length between 400 and 700 nanometers (1 nanometer is a billionth of meter.), electromagnetic waves of any other wavelength (like those used for TV transmission) have no reaction on our eyes. Early in the last century (1801) Thomas Young, who elucidated the wave nature of light, recognized that it is impossible to conceive that each sensitive part of retina contains an infinite number of particles, each capable of vibrating in unison with all possible wavelengths of light. He therefore proposed that the human eyes have three kinds of photoreceptors-- those corresponding to red, blue and green light. Even today scientists believe that there are only three kinds of photoreceptors. Then how do we perceive so many different colours? Each of these receptors are sensitive to the different wavelengths of visible light to varying extent, for example the red sensitive cone respond not only to red, but also, though less strongly, to orange yellow and green. The green sensitive cones respond also in some degree throughout the range red to blue.The blue-sensitive cones respond also to violet and green. Light of almost any single wavelength must excite at least two set of cone elements to some extent. When yellow light falls on the eyes it excites both red and green sensitive cones, so the effect is similar when red as well as green light falls on the eyes. How do we perceive white which is not