# NEET Physics Ray Optics and Optical Instruments Questions Solved

There  is an equiconvex glass lens with radius of each face as R and  and . If there is water in object space and air in image space, then the focal length is

(a)   2R                                      (b)   R

(c)   3 R/2                                  (d)  ${R}^{2}$

(c) Consider the refraction of the first surface i.e. refraction from rarer medium to denser medium

$\frac{{\mu }_{2}-{\mu }_{1}}{R}=\frac{{\mu }_{1}}{‐u}+\frac{{\mu }_{2}}{{v}_{1}}⇒\frac{\left(\frac{3}{2}\right)-\left(\frac{4}{3}\right)}{R}=\frac{\frac{4}{3}}{\infty }+\frac{\frac{3}{2}}{{v}_{1}}⇒{v}_{1}=9R$

Now consider the refraction at the second surface of the lens i.e. refraction from denser medium to rarer medium

$\frac{1-\frac{3}{2}}{-R}=-\frac{\frac{3}{2}}{9R}+\frac{1}{{v}_{2}}⇒{v}_{2}=\left(\frac{3}{2}\right)R$

The image will be formed at a distance of $\frac{3}{2}R$. This is equal to the focal length of the lens.

Difficulty Level:

• 21%
• 29%
• 43%
• 9%
Crack NEET with Online Course - Free Trial (Offer Valid Till September 17, 2019)