Question

When an object is kept at a distance of 30cm from a concave mirror, the image is formed at a distance of 10 cm. If the object is moved with a speed of `9cm s^(-1)` the speed with which the image moves is
A. `0.1ms^(-1)`
B. `1ms^(-1)`
C. `3ms^(-1)`
D. `9ms^(-1)`

Answer 1

Correct Answer - b.
`(1)/(v)+(1)/(u)=(1)/(f)`
`-(du)/(u^(2))-(dv)/(v^(2))=0` or `-(dv)/(u^(2))-(du)/(v^(2))`
or `dv=-(v^(2))/(u^(2))du`
`=-(10xx10)/(30xx30)xx9ms^(-1)=-1ms^(-1)`