Question

A thin rod of length 10 cm is placed along the axis of a concave mirror of focal length 30 cm such that its real diminished image just touches the rod. What is the length of the rod image ?

• A. 5cm
• B. 7.5 cm
• C. 10 cm
• D. 15 cm

Answer 1

Answer: (B)

7.5 cm

Answer 2

Here image is diminished so it is present between C and F, and one end of it lies on C.

For B end u = – 70 cm, f = – 30 cm

$\frac { 1 } { v } = \frac { 1 } { f } - \frac { 1 } { u } = \frac { 1 } { - 30 } - \frac { 1 } { - 70 } = - \frac { 4 } { 210 }$

v = – $\frac { 210 } { 4 }$ = – 52.5

size of image = A'B' = 60 cm – 52.5

= 7.5 cm