Question
Question: A concave mirror gives an image three times as large as the object placed at a distance of 20 cm fro...
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 should be
A
10 cm
B
15 cm
C
20 cm
D
30 cm
Answer
15 cm
Explanation
Solution
m = 
for real, 3 times magnified image,
m = – 3
– 3 = 
– 3 = 
or –3f – 60 = f
4f = – 60 or f = – 15 cm