Two small angled prisms A and B deviate the blue rays by `6^(@)` and `8^(@)` and the red rays by `4^(@)` and `6^(@)` respectively. Which prism has a greater dispersive power ?

Explanation

(a) Angular dispersion of A = `6^(@)-4^(@)=2^(@)`
Mean deviation of `A=(6+4)/2=5^(@)`
Dispersive power of A = `2/5` = 0.4
(b) Angular dispersion of B = `8^(@)-6^(@)=2^(@)`
Mean deviation of `B=(8+6)/2=7^(@)`
Dispersive power of B = `2/7` = 0.2857
Hence, dispersive power of A is greater than that of B.