. An object is placed at a distance of 20 cm in front of a concave lens of focal length 20 cm. find


An object is placed at a distance of 20 cm in front of a concave lens of focal length 20 cm. find: (a) the position of image, and (b) the magnification of image

 

Best Answer

Given,

Object distance, u = – 20 cm

Focal length, f = – 20 cm (concave lens)

(a) Lens formula is,

1 / v – 1 / u = 1 / f

∴ 1 / v = 1 / f + 1 / u

∴ 1 / v = 1 / – 20 + 1 / – 20

∴ 1 / v = – 2 / 20

∴ v = -10 cm

Hence the image is 10 cm in front of the lens on the same side as the object.

(ii) For a lens, magnification is

m = v / u

∴ m = -10 / – 20

∴ m = + 0.5

 

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