Question

A concave mirror has focal length 15 cm. Where should an object be placed infront of the mirror so that the real image formed is three times in size of the object ?

• A. 7.5 cm
• B. 10 cm
• C. 17.5 cm
• D. 20 cm

Answer 1

Answer: (D)

20 cm

Answer 2

f = –15cm u = ? m = ± 3

u = f = $\left( \frac { \pm 3 - 1 } { \pm 3 } \right)$ (–15)

For + ve u = – 10 cm

For – ve u = – 20 cm

For real image u = 20 cm