First slide

Ray optics

Question

A converging beam of light converges towards a point 20 cm behind an equiconvex lens placed in the path of the beam. After refraction through the lens, the beam converges at a distance of 10 cm from the lens. If refractive index of glass is 1.5, find the radius of curvature of the curved surfaces of the lens.

Easy

A

40 cm
B

30 cm
C

20 cm
D

10 cm

Solution

$\frac{1}{(+ 10)} - \frac{1}{(+ 20)} = \frac{1}{f} \Rightarrow \frac{1}{f} = \frac{1}{20} \Rightarrow f = 20 c m$
Now, $\frac{2 (μ - 1)}{R} = \frac{1}{f} \Rightarrow R = 2 (μ - 1) f = 2 (1.5 - 1) \times 20 c m$
$\Rightarrow R = 20 c m$

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