Question

A point object is placed $20cm$ left of a convex lens of focal length $f=5cm$ (see the figure). The lens is made to oscillate with small amplitude A along the horizontal axis. The image of the object will also oscillate along the axis with.

• A. Amplitude $A/9$, out of phase with the oscillations of the lens
• B. Amplitude $A/3$, out of phase with the oscillations of the lens
• C. Amplitude $A/3$, in phase with the oscillations of the lens
• D. Amplitude $A/9$, in phase with the oscillations of the lens

Answer 1

The correct option is A Amplitude $A/9$, out of phase with the oscillations of the lens

$\frac{1}{v}+\frac{1}{20}=\frac{1}{5}$
$v=\frac{20}{3}cm$
$△x_i=\frac{+v^2}{u^2}△x_0$
$△x_i=\frac{A}{9}$ out of phase with lens
Hence (A) is correct.