Determination of refractive index

  • Production of pure
  • Production of 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.

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.


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.


  1. List the three primary colours and the secondary colours formed from the mixture of these primary colours
  2. Explain the term ‘dispersion of light’
  3. (a) Define complimentary colours (b) List the primary colours and their corresponding compliments



  1. Which of the following colours of light is most deviated when light passes through a triangular prism? (a) orange (b) green (c) indigo (d) yellow
  2. Which of the following pairs of light rays shows the widest separation in the spectrum of white light? (a) green and blue (b) orange and indigo (c) blue and violet (d) red and yellow
  3. When a ray of sunlight passes obliquely through a rectangular glass block, (a) it emerges without displacement parallel to the incident ray (b) it gets dispersed into seven visible colours without any deviation at all (c) it deviates without dispersion (d) it gets laterally displaced, and the emergent ray is parallel to the incident ray
  4. The direction of a light ray changes as it passes from one medium to another. The phenomenon is called (a) diffraction (b) reflection (c) dispersion (d) refraction
  5. A piece of cloth appears green in sunlight. When held in red light, it will appear (a) green (b) blue (c) red (d) black
  6. The separation of white light into its constituent colour is known as (a) deviation (b)diffraction (c) dispersion   (d) deflection
  7. Another name for light of one wavelength or colour (a) dispersed light (b) hight light (c) monochromatic light (d) transparent light
  8. The following colours are primary colours except? (a)red (b) green (c)blue (d) yellow
  9. When white light passes through a triangular prism, there is dispersion because of (a) diffraction of light (b) polarization of light (c) the difference in speed of the components of light(d) the interference of light waves in glass
  10. The invisible part of the spectrum of white light consists of (a) infrared and ultraviolet only (b) infrared, ultraviolet and blue only (c) red, orange and green only (d) red and violet only


  1. Why does (a) green grass appear green and (b) red glass appear red when looked through in sunlight
  2. Describe with the aid of a well labeled diagram how a pure spectrum of white light can be produced?
  3. (a) Distinguish between pure and impure spectrum of white light (b) List the colours in the spectrum of white light in the descending order of wavelength

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