Question

An object is 20 cm away from a concave mirror and it is within the focal length of the mirror. If the mirror is changed to a plane mirror, the image moves 15 cm closer to the mirror.

Focal length of the concave mirror is -

• A. 36.6 cm
• B. 56.6 cm
• C. 66.6 cm
• D. 46.6 cm

Answer 1

Answer: (D)

46.6 cm

Answer 2

$\frac { 1 } { \mathrm { f } }$ = $\frac { 1 } { 20 }$ –

$\frac { 1 } { 20 }$ – $\frac { 1 } { \mathrm { f } }$ ; y =

y – 20 = 15 (given)

= 35

f = $\frac { 700 } { 15 }$cm