Question

A converging thin lens made out of crown glass (n=1.52) has a focal length f=1.0m An identical lens (same radius for the spherical surfaces), made out of a high-index plastic with n=1.70 will be ...

A. Converging with a focal length smaller than 1.0 m
B. Diverging with a focal length smaller than 1.0 m
C. Converging with a focal length exactly equal to 1.0 m
D. Converging with a focal length larger than 1.0 m
E. Diverging with a focal length larger than 1.0 m

Answer 1

Look up the lens maker's formula and decide for yourself.

The high-index plastic will have a shorter focal length than the glass lens with the same shape.

Focal length is inversely proportional to n-1.

Answer 2

oh k. got it. thanks