$A$ particle which has zero rest mass and non-zero energy and momentum must travel with a speed

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?

$A$ $60 \,W$ source emits monochromatic light of wavelength $662.5 \,nm$. The number of photons emitted per second is

Solar energy is incident normally on the earth's surface at the rate of about $1.4 \, kW m^{-2}$. The distance between the earth and the sun is $1.5 \times 10^{11} \, m$. Energy $E$ and mass $m$ are related by Einstein's equation $E = mc^2$,where $c = 3 \times 10^8 \, m s^{-1}$ is the speed of light in free space. The decrease in the mass of the sun is ........... $\, kg s^{-1}$.

$A$ photon and an electron, each of $20 \text{ eV}$ energy, move in free space. The ratio of linear momentum of electron $p_e$ to that of photon $p_{ph}$ is : (Take speed of light = $3 \times 10^8 \text{ ms}^{-1}$, charge of electron = $-1.6 \times 10^{-19} \text{ C}$ and mass of electron = $9 \times 10^{-31} \text{ kg}$)

What is the energy of a photon whose wavelength is $6840\,\mathring{A}$? (in $eV$)