When a ray of sun light passes through the prism each colour of light has its own speed in the glass. The seven colours come out of the prism with different angles of deviation. Red deviates least and violet deviates maximum. Thus a prism can disperse a white light into seven colours which is called colour spectrum. For a thin prism the angle of deviation `delta` =(n-1) A deviation is given. The angle of dispersion is given as `theta`= `delta v`-`delta r`. The dispersive power is `omega` = `theta`/ `delta mean` Using the above ideas give answer the following questions A prism has R.I. for violet and red ” =1.523 n =1.5145 If the angle of prism is A= 2° the dispersive power of the prism is : 0.01639 1.593 0.1639 0.18