Question

When an object is kept at a distance of 30 cm from a concave mirror, the image is formed at a distance of 10cm. If the object is moved with a speed of 9, the speed with which image moves is:

• A. Unknown text
• B. Unknown text
• C. Unknown text
• D. Unknown text

Answer 1

Answer: (B)

Unknown text

Answer 2

: According to mirror formula,

Differentiating with respect to t, we get

or $\frac { \mathrm { dv } } { \mathrm { dt } } = - \left( \frac { \mathrm { v } } { \mathrm { u } } \right) ^ { 2 } \frac { \mathrm { du } } { \mathrm { dt } }$

$\mathrm { v } _ { \mathrm { i } } = - \left( \frac { \mathrm { v } } { \mathrm { u } } \right) ^ { 2 } \mathrm { v } _ { 0 } \left( \because \frac { \mathrm { dv } } { \mathrm { dt } } = \mathrm { v } _ { \mathrm { i } } \right.$ and $\left. \frac { \mathrm { du } } { \mathrm { dt } } = \mathrm { v } _ { \mathrm { o } } \right)$

Substituting thegiven values, we get

$\left| \mathrm { v } _ { 1 } \right| = 1 \mathrm {~ms} ^ { - 1 }$