A lens of focal length 20.0 cm and aperture radius 2.0 cm is placed at a distance 30.0 cm from a point source of light. On the other side a screen is placed at a distance 50.0 cm from the lens. The radius of spot of light formed on screen is (neglect spherical abberation through lens)

1/2 cm

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1/3 cm

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1/5 cm

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1.0 cm

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Solution

The correct option is B 1/3 cm
Finding the image after refraction from the lens,
$\frac{1}{v} - \frac{1}{- 30} = \frac{1}{20}$
hence, $v = 60 c m$
As shown in the figure, a circle of radius h would be obtained due to the screen placed between image and lens.
From simple geometry, we see
$\frac{h}{10} = \frac{2}{60}$
Hence $h = \frac{1}{3} c m$

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A lens of focal length 20.0 cm and aperture radius 2.0 cm is placed at a distance 30.0 cm from a point source of light. On the other side a screen is placed at a distance 50.0 cm from the lens. The radius of spot of light formed on screen is (neglect spherical abberation through lens)

The correct option is B 1/3 cmFinding the image after refraction from the lens,1v−1−30=120hence, v=60cmAs shown in the figure, a circle of radius h would be obtained due to the screen placed between image and lens. From simple geometry, we seeh10=260Hence h=13cm