In fundamental mode, the time required for the sound wave to reach up to closed end of a pipe filled with air is ' $t$ ' second. The frequency of vibration of air column is (Neglect end correction)

A wire has three different sections as shown in figure. The magnitude of the magnetic field produced at the centre ' $O$ ' of the semicircle by three sections together is ( $\mu_0=$ permiability of free space)

A student measures time for 20 oscillations of a simple pendulum as $30 \mathrm{~s}, 32 \mathrm{~s}, 35 \mathrm{~s}$ and 35 s . If the minimum division in the measuring clock is 1 s , then correct mean time (in second) is

Light of wavelength ' $\lambda$ ' falls on a metal having work function $\frac{\mathrm{hc}}{\lambda_0}$. Photoelectric effect will take place only if ( $\lambda_0$ is the threshold wavelength)