The ratio of the accelerating potentials required to accelerate (i) an $\alpha$-particle and (ii) a proton to have the same de Broglie wavelength associated with them is
(mass of $\alpha$ - particle = $6.4 \times 10^{-27}$ kg, mass of proton = $1.6 \times 10^{-27}$ kg)

In photoelectric effect, keeping the frequency of incident radiation and accelerating potential fixed, if the intensity of incident light is increased,

In Bohr's atomic model, the energy of the electron is 'E' in the second orbit of hydrogen atom. So the energy of the electron in the third orbit of helium atom $(Z = 2)$ will be

The activity of a radioactive sample is measured as $N_0$ counts per minute at time $t = 0$ and $\dfrac{N_0}{e}$ counts per minute at time $t = 3$ minute. The time (in minute) in which the activity reduces to half the value, is