A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is

  1. – 30 cm
  2. – 20 cm
  3. – 40 cm
  4. – 60 cm

Answer

Here, size of object = O = + 10.0 mm = + 1.0 cm (as, 1 cm = 10 mm)

Size of Image size = I = 5.0 mm = 0.5 cm                                                                             

Image distance, v = − 30 cm   (as image is real)

Let, object distance = u

Focal length, f =?

 

The correct option is B.

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