35. To form a real point size image of a very far off object at a distance of 20 cm from the lens, focal length of lens should be

(a) 20 cm

(b) 80 cm

(d) 100 cm

Convex lenses can converge the rays coming from long distance at their focal length and forms a point sized image.

For the above mentioned condition, we can use a convex lens of focal length 20cm so that it can form image of point size at 20cm.

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Given: Object is placed at a distance of 20cm from the lens.

To Find: focal length in order to form a real point size image.


The focal length of a lens is defined as the distance from the lens at which the light rays from infinity converge after refraction through the lens.

Also, the lens equations states that,


  •  = focal length of the lens.
  •  = distance of the object from the lens.
  •  = distance of the image formed from the lens.

    Given that object is very far away from the lens, such that,

    Using lens equation,


    Also, since the image formed is real and point sized which means the lens is convex, therefore the focal length should be positive too.

    Therefore, focal length of the lens = +20 cm.                                                                                                                                                                                                 Regards,

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