Question

The diameter of aperture of a plano-convex lens is 6 cm and maximum thickness is 3 mm. If velocity of light in material is $2\times$10${}^8$ m/s, calculate focal length of the lens :

• A. $10cm$
• B. $20cm$
• C. $15cm$
• D. $30cm$

Answer 1

The correct option is D $30cm$

The radius of curvature of the lens is,

$R=\frac{r^2}{2x}$

where,$r=3$ , $x=0.3$

Thus, $R=15$

Also, $\mu =\frac{3\times {10}^8}{2\times {10}^8}$

$\mu =1.5$

Now, $f=\frac{R}{\mu -1}$

Thus, $f=\frac{15}{0.5}=30cm$