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Dispersion of light

This unit explains everything you need to know about Dispersion of light

This is the separation of white light into various colours listed in order. The colours are red, orange, yellow, green, blue, indigo, and violet. The bundle of colours formed is called a spectrum.

Visible light spectrum can be made by passing a beam of white light through a glass prism.

Dispersion occurs because each colour is refracted in glass by different amount i.e. each colour has different refractive index. So red is refracted least and violet is refracted most.

How obtain a pure spectrum

The spectrum obtained above is impure i.e. the colours of the spectrum overlap one another
Apure spectrum is one in which light of one colour only forms each part of the image on the screen without overlap.

This can be achieved by placing a convex lens in front of the prism to increase on the deviation of the colours as they pass through the prism.

Lens L produces parallel beam of white light. The light is then dispersed and deviated at the prism sprinting up into various colours.
Lens B collects the different coloured lines so that the parallel beam of each separate colour is focused on the screen.

Recombination of the spectrum:

The colours of the spectrum can be recombined by;
(i) Arranging a second prism so that the light is deviated in the opposite direction.

(ii) Using an electric motor to rotate at high speed, a disc with spectral colours from its sectors as shown below.

The white light is slightly grey because paints are not pure colours.
Colours of objects:
The colour of an object depends on;
 The colour of light falling on it.
 The colour it transmits or reflects e.g. an object appears blue because it reflects blue light into
the eyes and absorbs the other colours of the spectrum. Similarly, an object appears red
because it reflects red light into the eyes and absorbs all other colours.
 A white object reflects all the colours of the spectrum into the eyes and absorbs none.

Types of colours:
a. Primary colours
These are colours that can’t be obtained by adding two different colours of light. They include red ,
blue and green
b. Secondary colours
These are colours which are obtained by adding 2 primary colours together. They include yellow,
peacock blue and magenta.
NB:- peacock blue is at times called cyan or tachois.
c. Complementally colours
These are two different colours which when added produce white light. One of them is a secondary
colour and the other must be a primary colour. The pairs are
Red + peacock blue white light
Green + magenta white light
Blue + yellow white light
From the complementally colours it is noted that when the three primary colours are joined, they produce white light.

Summary of coloured lights

Coloured objects in white light
A coloured object reflects and transmits its own colour and absorbs other colour incident on it.

N.B:- primary colour +primary colour = black
Primary colour + secondary colour = primary
Secondary colour + secondary colour = common primary colour.
Question 1
Describe and explain the appearance of a red tie with blue spots when observed in;
a. Red light
b. green light
a) Green light – the red tie appears black because both colours are primary colours and non is reflected

b) Red light – in the red light the tie appears red and blue spots appear black.This is
because the red reflects the red colour and observes blue colour.
Question 2
A plant with green leaves and red flowers is placed in
a) green
b) blue
c) Yellow
What colour will the leaves and flowers appear in each case . Assume all colours are pure
a. green -: the leaves remain green but the flowers black
b. blue -: the leaves will appear black and flowers black
c. Yellow -: the leaves appear green and flowers appear red

Filters (colour)

A filter is a coloured sheet of plastic or glass material which allows light of its own type to pass through it and absorbs the rest of the coloured lights i.e. a green filter transmits only green, a blue transmits only blue, a yellow filter transmits red, green and yellow lights.

Mixing of coloured pigments

A pigment is a substance which gives its colour to another substance .A pigment absorbs all the
colours except its own which it reflects. When pigments are mixed, the colour reflected is the
common to all e.g. blue + yellow green
Yellow + orange black
Green + indigo blue
The blue reflects indigo and green, its neighbour in the spectrum as well as blue
Yellow reflects green, yellow and orange, only green is reflected by both

Mixing coloured pigments is called colour mixing by subtraction
Pigments appear black because none of the colours are reflected