In Ramsden eyepiece, the two planoconvex lenses each of focal length f are separated by a distance 12 cm. The equivalent focal length (in cm) of the eyepiece is

Options

A. 10.5
B. 12
C. 13.5
D. 15.5

Answer

C

Explanation

The distance between two piano convex lens is given by,
`d=(2f)/3`
`implies = (3d)/2=(3xx12)/2 =18cm`
` therefore "Equival ent focal length" =3/4 f=3/4 xx18=13.5 cm`