First slide

Ray optics

Question

A convex lens is at a distance of 10 cm from a convex mirror of focal length 10 cm as shown in figure. A beam of light parallel to the principal axis is incident on the tens. After refraction through the lens, reflection at the mirror and again refraction through the lens, the final emergent beam refraces the path of the incident beam then focal length of the lens is

Moderate

A

60 cm
B

40 cm
C

20 cm
D

30 cm

Solution

In this case rays of light emerging from the lens must strike the mirror perpendicularly. Therefore C must be the second focal point of the lens and the centre of curvature of the mirror.
$∴ f_{l} = 10 + 2 f_{m} = (10 + 20) cm = 30 cm$

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