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Question

A 2 cm long pin is placed perpendicular to the principal axis of a convex lens of focal length 15 cm at distance of 25 cm from the lens. Find the position of image and its size. [3 MARKS]

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Solution

Lens formula: 1 Mark
Position and size of image: 1 Mark each

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

Using the Len's formula, 1v1u=1f we get

2. 1v125=1+15

v=37.5cm

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

3. Magnification is given by

m=hh=vu

hh=37.525=1.5

h=1.5×h=1.5×2cm=3cm

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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