Question

A concave mirror gives an image three times as large as the object placed at a distance of 20 cm from it. For the image to be real, the focal length of mirror should be –

• A. – 20cm
• B. – 15cm
• C.
  • 20 cm
• D. +15 cm

Answer 1

Answer: (B)

– 15cm

Answer 2

m = – 3 = $\frac { f } { f - u }$ ; – 3 = $\frac { f } { f - ( - 20 ) }$

– 3f – 60 = f ; – 4f = 60

Ž f = – 15 cm