Question

A point object is placed 20 cm left of a convex lens of focal length f = 5 cm (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 $\frac{A}{9}$, out of phase with the oscillations of the lens
• B. amplitude $\frac{A}{3}$, out of phase with the oscillations of the lens
• C. amplitude $\frac{A}{3}$, in phase with the oscillations of the lens
• D. amplitude $\frac{A}{9}$, in phase with the oscillations of the lens

Answer 1

The correct option is A

amplitude $\frac{A}{9}$, out of phase with the oscillations of the lens

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