Question

A parallel beam of light falls one a thick lens of thickness 4 cm and refractive index 1.5 as shown in the figure. The surface radii are $R_1$ = 12 and $R_2$ = 16 m. The position of focal point on the optical axis of the lens from the centre O is

• A. 12.8cm to the right of O
• B. 13.8cm to the left of O
• C. 14.8cm to right of O
• D. 16.8cm to the right of O

Answer 1

The correct option is C 14.8cm to right of O

For the first refraction
$\frac{1.5}{v}-\frac{1}{\infty }=\frac{1.5-1}{12}\Rightarrow v=36cm$
$\therefore$ For the second refraction u = 36 – 4 = 32 cm
$\frac{1}{v}-\frac{1.5}{32}=\frac{1-1.5}{-16}$
$\Rightarrow v=12.8cm$
$\therefore$ Distance from 0 = 12.8+2=14.8 cm to the right of O.