Question

A concave mirror gives an image three times as large as the object placed at a distance of 20cm from it. For the image to be real, the focal length should be
(a) 10cm
(b) 15cm
(c) 20cm(d) 30cm

Answer 1

Answer:

Explanation:

=> According to the question, a concave mirror gives an image three times as large as the object placed at a distance of 20cm from it.

Thus, object distance, u = -20cm

As image is real. thus, m = -3

=> As per the Megnification formula for mirror :

m = -v/u

-3 = -v/(-20)

-v = -3 * -20

v = -60 cm  

According to Mirror formula:

1/v + 1/u = 1/f

1/(−60) +1/ (−20) = 1/f

∴ 1/f= −1−3/60

=−4/60  

f=−15 cm

Thus, For the image to be real, the focal length should be -15 cm

Answer 2

Hope it helps u....