First slide

Ray optics

Question

The radius of curvature of curved surface of a thin plano - convex lens is 10 cm and the refractive index is 1.5. If the plane surface is silvered, then the focal length will be

Easy

A

15 cm
B

20 cm
C

5 cm
D

10 cm

Solution

$R = 10 c m P l a n o c o n v e x l e n s μ = 1.5$
$\begin{array}{l} \frac{1}{f_{l}} = (μ - 1) (\frac{1}{R}) \\ \frac{1}{f_{l}} = (0.5) (\frac{1}{10}) = \frac{1}{20} \end{array}$
When silvered at flat surface
$\begin{array}{l} \frac{1}{f} = \frac{2}{f_{l}} + \frac{1}{\infty} \\ \frac{1}{f} = \frac{2}{f_{l}} = \frac{2}{20} = \frac{1}{10} f = 10 c m \end{array}$

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