Which of the following has a mass closest in value to that of the positron? $(1\;a.m.u = 931\;MeV)$

Light of intensity $10^{-10} \ W/m^2$ and wavelength $5.6 \times 10^{-7} \ m$ is incident on a surface of area $10^{-6} \ m^2$. What is the number of photons incident per second?

An electron and a photon have the same wavelength of $10^{-9} \, m$. If $E$ is the energy of the photon and $p$ is the momentum of the electron,then the magnitude of $E / p$ (in $SI$ unit) is

The average number of photons emitted per second by a laser of power $6.6 \times 10^{-3} \,W$ producing light of wavelength $600 \,nm$ is (Planck's constant,$h = 6.6 \times 10^{-34} \,J \cdot s$):

The ratio of energies of photons with wavelengths $\lambda_1 = 150 \ nm$ and $\lambda_2 = 300 \ nm$ is:

There are $n_1$ photons of frequency $\nu_1$ in a beam of light. In an equally energetic beam,there are $n_2$ photons of frequency $\nu_2$. Then the correct relation is