Rays from Sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object?

  1. 15 cm in front of the mirror
  2. 30 cm in front of the mirror
  3. between 15 cm and 30 cm in front of the mirror
  4. more than 30 cm in front of the mirror


Sun is at infinite distance from the surface of the earth. Now light rays from sun rays after reflection from concave mirror will converge at focus.

It is given that sun rays converge at 15 cm. So, focal length of the mirror = f = 15 cm.

In case of a concave mirror, the size of image and object will be same if the object is placed at 2f.

Hence, in this case object must be place at 2f or 2 × 15 = 30 cm.

The correct option is B.

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