Question

The focal length of a glass lens in air is $f$. When immersed in water (refractive index $u$) its focal length would be_______.

(a) $f$

(b) less than $f ~(<f)$

(c) greater than $f ~(>f)$

(d) nothing can be decided​

Answer 1

Answer:

Since the relative refractive index of glass with respect to water is less than that of glass with respect to air, the bending of light at the interface of glass water is lesser and hence a parallel beam of light falling on the lens is bent less, to meet at point on the principal axis and hence the lens has more focal length.

OPTION (C)