A ray of light falls on the surface of a spherical glass paper weight making an angle `alpha` with the normal and is refracted in the medium at an angle `beta`. The angle of deviation of the emergent ray from the direction of the incident ray is :

Explanation

Deviation means the angle through which the incident ray is turned in emerging from the medium. In Figure if AB and DE are the incident and emergent rays respectively, the deviation will be `delta`.
Now as at B, `/_ i=alpha` and `/_r = beta`
So from Snell’s law, 1sin `alpha` = `mu` sin `beta` ....(1)
Now from geometry of figure at D, `/_i = beta`
So `musingamma` =1 sin `gamma` ................ (2)
Comparing Eqs. (1) and (2) `gamma = alpha`
Now as in a triangle exterior angle is the sum of remain-ing two interior angles, in `DeltaABCD`,
`delta=(alpha-beta)+(alpha-beta)=2(alpha-beta)`