KCET 2014 Physics Solved Paper

Given, position of object = u = 20 cm; image formed = 3 times magnified and real
Using m=

−v

m

,we get
3=‌

−v

20

⇒v=−60cm
Using mirror formula,

1

f

=

1

v

+

1

w

,we get
Therefore, focal length of concave mirror = 15 cm