. A concave lens forms the image of an object kept at a distance 20 cm in front of it, at a distance 10 cm on the side of the object


A concave lens forms the image of an object kept at a distance 20 cm in front of it, at a distance 10 cm on the side of the object.

(a) What is the nature of the image?

(b) Find the focal length of the lens.

 

Best Answer

Given,

Object distance, u = -20 cm

Image distance, v = – 10 cm

(a) It is a virtual image because the image is formed on the same side as the object. Also, since the lens is a concave lens the image will be erect and diminished.

(b) Lens formula is

1 / v – 1 / u = 1 / f

∴ 1 / -10 – 1 / -20 = 1 / f

∴ 1 / f = 1 / 20 – 1 / 10

∴ 1 / f = – 1 / 20

∴ f = – 20 cm

 

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