Question

Please answer this
from jee 2020​

Answer 1

Explanation:

w.k.t

1/f=1/u+1/v--------(1)

where u is object distance

v is image distance

Now differentiate (1) w.r.t t,we get

- 1/u^2 du/dt - 1/v^2 dv/dt=0

dv/dt = -(v^2/u^2) du/dt

dv/dt= -(100/900)×9

dv/dt= -1 cm/s