The ratio of the frequencies of two simple pendulums is $4: 3$ at the same place. The ratio of their respective lengths is

A source of sound emits sound wave of frequency ' f ' and moves towards an observer with a velocity $\frac{\mathrm{V}}{3}$ where V is the velocity of sound. If the observer moves away from the source with a velocity $\frac{\mathrm{V}}{5}$ the apparent frequency heard by him will be

In Young's double slit experiment, the distance between the slits is 2 mm and the slits are 1 m away, from the screen. Two interference patterns can be obtained on the screen due to light of wavelength ' $\lambda_1$ ' and ' $\lambda_2$ ' respectively. The separation on the screen between the $3^{\text {rd }}$ order bright fringes on the two interference patterns is ( $\lambda_2=1.5 \lambda_1$ )

$\mathrm{L}, \mathrm{C}$ and R are connected in series to an a. c. source. Which one of the following is true?

Phase relation between current and voltage is such that