DISPERSION OF LIGHT AND COLOURS

 

DISPERSION OF LIGHT AND COLOURS

COLOUR OF LIGHT.

It was observed that when white light is passed through a prism, an elongated colour patch of light is obtained on a screen placed behind the prism. The colour pattern is known as the spectrum of white light. The spectrum consists of the Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV), in that order from the apex side of the prism.

Recombination of Spectrum

When another identical prism is placed to intercept the refracted rays in the same manner , the same arrangement of colours emerge on the screen, but this time it is found that the colours are more widely separated.

If the second prism is inverted, the colours are seen to disappear and only a patch of white light will be visible on the screen. The disappearance of the colours was due to their recombination to reproduce white light.

 DISPERSION

This is the separation of white light into its component colours of Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Dispersion is due to the fact that the different colours of white light travel at different speeds through the glass. Each colour is therefore refracted in a slightly different direction or angle through the glass. Careful observation of the spectrum shows that the red light is deviated list and the violet is deviated most. Therefore red light travels in glass with the greatest speed.

The rainbow is formed by the dispersion of sunlight by spherical raindrops floating in the air.

Monochromatic Light

This is the light of one wavelength. When such light of one colour or wavelength is passed through a prism, refraction occurs without dispersion.

 Pure spectrum: is that the spectrum produced by prism in which the colours are clearly separated or distinct from each other.

 Impure spectrum: the spectrum produced by prism when the different colours overlap.

 Forms Of Colours

The colour of an object depends on the colours in the incident or falling on it, and also on the absorption and reflection of this light by the object. A white light appears white in daylight because it reflect equally all the colours of the spectrum. If a red light is incident on it, it appears red. A black object in daylight absorbs all the colours of the spectrum and reflects none at all. The green leaf looks dark or black if it is illuminated in a dark room by blue light.

Additive Mixing Of Colours  (Colour triangles and circles)

We can obtain a variety of colours by mixing the different colours of the spectrum. We cannot however, obtain Red, Green and Blue colour by mixing other colours. These three colours are therefore called the primary colours. The adding of primary colours to produce other colours is known as additive colour mixing or additive combination of colours. Secondary colours are the colours we obtain by mixing any two of the primary colours. Example of such mixing are: red + green = yellow; red + blue = magenta ( a purple colour); green + blue =  cyan (turquoise or blue-green colour). Yellow, magenta and cyan are the secondary colours. All the three primary colours combine to produce white (red + green + blue = white).

A useful way of remembering the addition of colours is by the colour triangle. Each primary colour is placed at the corner of an equilateral triangle RGB. Each secondary colour is at the midpoint of the sides RB, BG and GR, and represent the colour obtained by mixing or adding equal quantities of primaries at  the corners. The centre o of the triangle represents white light obtained by the addition of equal quantities of the three primary colours. A primary colour when mixed with an appropriate secondary colour, that is, the colour directly opposite to it in the colour triangle, will produce white. For example, Green + Magenta = White; Blue + Yellow = White; Red + Cyan =White. Such colours which produce white light when mixed in a suitable proportions are called complementary colours.