Question

A real image formed by concave mirror is 4.5 times the size of the object. If the mirror is 20 cm from the object, its focal length is –

• A. $\frac { 90 } { 11 }$ cm
• B. $\frac { 120 } { 11 }$cm
• C. $\frac { 150 } { 11 }$cm
• D. $\frac { 180 } { 11 }$cm

Answer 1

Answer: (D)

$\frac { 180 } { 11 }$cm

Answer 2

m = –4.5 = –

u = f Ž f = $\frac { \mathrm { mu } } { ( \mathrm { m } - 1 ) }$ = $\frac { \left( - \frac { 9 } { 2 } \right) ( - 20 \mathrm {~cm} ) } { \left( - \frac { 9 } { 2 } - 1 \right) }$

= $\frac { 90 } { - \frac { 11 } { 2 } }$= $- \frac { 180 } { 11 }$ cm