The fundamental frequency of a closed pipe is 400 Hz . If $\frac{1}{3}$ rd pipe is filled with water, then the frequency of 2nd harmonic of the pipe will be (neglect and correction)

An AC circuit contains resistance of $12 \Omega$ and inductive reactance $5 \Omega$. The phase angle between current and potential difference will be

In an intrinsic semiconductor, at an ordinary temperature, the correct relation between the number of electrons per unit volume $n_e$ and number of holes per unit volume $n_h$.

By increasing the aperture of the objective lens, wavelength of light, focal length of the objective lens and the resolving power of an astronomical telescope respectively