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Question

A 2 cm long pin is placed perpendicular to the principal axis of a lens of focal length 15 cm at distance of 25 cm from the lens. Find the size of the image.


A

The image of the pin is 3 cm long.

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B

The image of the pin is 2 cm long.

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C

The image of the pin is 5 cm long.

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D

The image of the pin is 10 cm long.

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Solution

The correct option is A

The image of the pin is 3 cm long.


Here, u = - 25 cm, f = +15

Using the Len's formula, 1v - 1u = 1f we get

1v - 125 = 1+15

v = 37.5 cm

The positive sign shows that the image is formed on the right -hand side of the lens.

Magnification is given by

m = h/h = vu

h/h = 37.525 = - 1.5

h = - 1.5 × h = - 1.5 × 2 cm = - 3 cm

The image of the pin is 3 cm long. The negative sign shows that it is formed below the principal axis,i.e the image is inverted.


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