Question

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 ?

Answer 1

(a) Angular dispersion = `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.