Notes Physics

Dispersion of white light, production of pure and impure spectrum

White light has a band of wavelengths of different colours.  This is called the spectrum colours.  Red light has the longest wavelength in air (700 x 10-9m) while violet light has the shortest wavelength (450 x 10-9m) in air.

In a vacuum and in air, all the colours of white light travel at the same speed.  But in glass, the colours travel at different speeds.Thus, a glass prism can separate or dispute white light into its various colours or wavelengths.

What is Dispersion? The white light is separated in different colors to  form the rainbow is named dispersion. #dispersion… | White light, Dispersion  of light, Light

White light from a source e.g sunlight, passes through a narrow slit and is incident on the glass prism. After leaving the glass prism, white light is separated into a band or spread of impulse colours which are formed on the screen.  The spectrum of white light consists of (bands of) red, orange, yellow, green, blue, indigo and violet colours (ROYGBIV).  The separation of the colours by the glass prism is called dispersion.  The red colour is deviated least, while the violet colour is deviated most.

Production of a pure spectrum

The spectrum described above is an impure spectrum, because the different bands of colour overlap.  A spectrum in which such an overlap does not occur is called a pure spectrum.  This can be obtained by using an arrangement of converging lenses in addition to the glass prism.

White light from a source passes through a narrow slit and are incident on the first converging lens.  The slit is located at the focus of the lens, and hence the white light is rendered parallel after refraction through the lens.  Thus, a beam of parallel light is incident on the glass prism. In this way, rays of the same colour will suffer the same amount of deviation by the prism, and each colour will emerge as a parallel beam.  They are then brought to focus by the second converging lens.  The different colours, red, orange, yellow, green, blue, indigo and violet are then brought to different foci on the screen.


Each colour of light has its own characteristic wavelength.  If the light if the yellow wavelength enters the eye, it sees yellow.  However, if a mixture of red and green light enters the eye it also sees yellow.  All the colours that the eye sees can be made by mixing three basic colours, these three colours, which are called primary colours, are red, blue and green.

The colour made by mixing any two primary colours are called secondary colours.  These are:

(i)         red + blue = magenta

(ii)        blue + green = cyan

(iii)       green + red = yellow

The mixing of coloured lights is known as additive mixing.  All the three primary colours mix together to give white light.

Red + blue + green = white

The operation of colour movies is based on addictive colour mixing.


Coloured filters are made out of coloured glass. A coloured filter transmits its own colour, but absorbs any other colour which falls on it.


An object can only be seen when light is reflected from it into the eye.  The substance which gives an object its colour is called a pigment.  A pigment absorbs all colours except its own, which it reflects.

A black pigment absorbs all colours and reflects none. A white pigment reflects all colours.  Coloured objects such as pigments (paints) used by painters can also be mixed together. The mixing of colours pigments is known as subtractive mixing.

General Revison

  1.  Displacement of a particle executing SHM is given by    x = 0.01 sin (100 πt + 0.05). Its time period is …………..
  2.  A force of 6.4 N stretches a vertical spring by 0.1 m. The mass  that must be suspended from the spring so that it oscillates with  a period of s is………………

Weekend Assignment

1. The separation of white light into its constituent colour is known as

(a) deviation (b)defraction   (c) dispersion   (d) deflection

2. The light of one wavelength or colour is called

  (a) yellow light  (b) green light  (c) monochromatic light  (d) blue light

3. The colours obtained by mixing any two primary colours are called ……..

  (a) primary colours  (b) secondary colours  (c) indigo    (d) violet

4.  In a pure spectrum, what is the function of the lens near the length source?

   (a) to separate the light colours        (b) to produce parallel rays (c) to diverge the light rays         (d )to produce dispersion necessary for the spectrum

5. The following colours are primary colours except?       (a )Red     (b) Green (c)Blue  (d) Yellow


  1.  Explain the term ‘dispersion’
  2. Describe with the aid of a well labeled diagram how a pure spectrum of white light can be produced?

Reading Assignment

New School physics for senior secondary schools pages 301 – 305

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