The refractive indices of the crown glass for violet and red lights are`1.51`and `1.49`respectively and those of the flint glass are`1.77`and`1.73`respectively A prism of angle `6^(@)`is made of crown glass .A beam of white light is incident at a small angle on this prism.The other thin flint glass prism is combined white the crown glass prism such that te net mean deviation is`1.5^(@)`anticlockwise.
(i)Determine the angle of the flint glass prism.

(ii)A screen is placed normal to the emerging beam at a distance of`2m`from the prism combination.find the distance between red and violet spot on the screen Which is the topmost colour on screen.

Explanation

Crown glass: `mu_b=1.51`, `mu_r=1.49`, `A=6^@`
Flint glass: `mu_b^'=1.77`, `mu_r^'=1.73`, `A^'=?`
Crown glass, `mu_y=(mu_b+mu_r)/(2)=1.50`
Flint glass, `mu_y^'=(mu_b^'+mu_r^')/(2)=1.75`
For no deviation: `(mu_r-1)A=(mu_y^'-1)A^'`
`(1.50-1)xx6=(1/75-1)A^'`
`A^'=4^@`
Dispersion: `theta_0=theta_1-theta_2=|(mu_b-mu_r)A-(mu_b^'-mu_r^')A^'|`
`=|(1.51-1.49)xx6-(1.77-1.73)xx4|`
`=|0.12-0.16|`
`=0.04^@`