Question

A concave lens made of material of refractive index 1.6 is immersed in a medium of refractive index 2.0. The two surface of the concave lens have the same radius of curvature 0.2 m. The lens will behave as a

• A. Divergent lens of focal length 0.4m
• B. Divergent lens of focal length 0.5 m
• C. Convergent lens of focal length 0.4m
• D. Convergent lens of focal length 0.5m

Answer 1

Answer: (D)

Convergent lens of focal length 0.5m

${\mu }_2=1.6$

${\mu }_1=2.0$

$R_1=R_2=0.2m$

So from lensmaker's formula

$\frac{1}{f}=\frac{-1}{5}(\frac{-1}{0.2}-\frac{1}{0.2})$

$f=0.5m$

hence a convergent lens.