A concave spherical surface of radius of curvature `R = 20 cm` separates two media A and B having refractive indices `mu_(A) =(4)/(3)` and `mu_(B) =(3)/(2)`respectively. A point object is placed on the principal axis. Find the distance of the object from the surface so that its image is virtual when
(a) the object is in medium A.
(b) the object is in medium B.

Options

A. image is always real
B. image is real if the object distance is greater than `90cm`
C. image is always virtual
D. image is virtual if the object distacne is less than`90cm`

Answer

C

Explanation

`(mu_(2))/(v)-(mu_(1))/(u)=(mu_(2)-mu_(1))/R `
`(3)/(2v)-(4)/(3u)=(3//2-4//3)/(R)`
`(3)/(2v)+(1)/(6R)+(4)/(3u)`
since `R`and `u` both are `-ve`so v will be -ve image will always be virtual