Question

An object is placed in front of a concave mirror of focal length 20 cm. The image formed is 3 times the size of the object. Calculate the two possible distances of the object from the mirror.

Answer 1

Object-size, h = + 5.0 cm;
Object-distance, u = – 20.0 cm;
Focal length, f = –15.0 cm;
Image-distance, v = ?
Image-size, h′ = ?
Mirror formula is             (1/v) + (1/u) = 1/f

1/v = (1/f) − (1/u)
      = (−1/15) + (1/20)
      = −1/60
v = − 60 cm.

The screen should be placed at 60 cm from the mirror. The image is real.

Magnification m = h'/h = − v/u = 60/(– 20) = – 3

Height of the image h' = mh = (– 3)x5 = − 15 cm

The image in inverted and enlarged