First slide

The lens

Question

The aperture radius of thin convex lens is 2 mm. Find the radius of circular bright spot on screen

Difficult

A

1 mm
B

2 mm
C

3 mm
D

4 mm

Solution

For lens, $u = - 30 c m, f = 20 c m$
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f} o r \frac{1}{v} + \frac{1}{30} = \frac{1}{20} v = + 60 c m$
From figure, $\tan θ = \frac{2 m m}{60 c m} = \frac{r}{30 c m}$
$∴ r = 1 m m$

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