A screen is placed $90 c m$ from an object. The image of an object on the screen is formed by a convex lens at two different locations separated by $20 c m$ . The focal length of the lens is :

18 c m

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21.4 c m

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60 c m

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85.6 c m

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Solution

The correct option is C $21.4 c m$
It is an example of the displacement method of determining the focal length of a convex lens.

Screen Distance from Object is $D = 90 c m$ and shift in position of lens is $x = 20 c m$

Focal length, $f = \frac{D^{2} - x^{2}}{4 D} = 7700 / 360 = 21.388 c m$
Option B is correct.

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A screen is placed 90cm from an object. The image of an object on the screen is formed by a convex lens at two different locations separated by 20cm. The focal length of the lens is :

The correct option is C 21.4cmIt is an example of the displacement method of determining the focal length of a convex lens.Screen Distance from Object is D=90cm and shift in position of lens is x=20cmFocal length, f=D2−x24D=7700/360=21.388cm Option B is correct.

A screen is placed $90 c m$ from an object. The image of an object on the screen is formed by a convex lens at two different locations separated by $20 c m$ . The focal length of the lens is :