A beam of polychromatic light passes through a thin prism of prism angle 6 . The refractive index of the material of the prism varies with wavelength ( ) as n( )= + 2, where =3 m-1 and =0.096 m2. If is the wavelength at which the angle of minimum deviation Dm is smallest, then the correct value of Dm at is
- 6.4
- 4.8
- 3.2
- 2.4
Answer
(2) 4.8
Solution
Key idea: thin prism Dm=(n-1)A, so Dm is smallest when n( ) is minimum.
dnd = - 2 3=0 3= 2 0.0963=0.064 =0.4 .
n=3 0.4+ 0.0960.16=1.2+0.6=1.8.
Dm=(1.8-1) 6 =4.8 .
Answer: (B)