Question

An eye piece is constructed so that spherical and chromatic aberrations are minimized to the maximum extent. If its equivalent focal length is $P$ then the focal length of the field lens is

• A. $3P/2$
• B. $2P$
• C. $2P/3$
• D. $3P$

Answer 1

The correct option is B $2P$

An eye-piece with minimum spherical and chromatic aberration than it is Huygens's eye-piece.
In Huygens's eye-piece equivalent focal length is $\frac{3\delta }{2}$ and focal length of field lens is $3\delta$.
So, focal length of field lens is twice of equivalent focal length.
So, focal length of field lens is $2P$.