If the mean wavelength of light radiated by a $100 \ W$ lamp is $5000 \ \mathring{A}$,then the number of photons radiated per second is:

Consider two black bodies $A$ and $B$ having equal surface areas. On the surface of $A$, $n$ photons of frequency $f$ are incident perpendicularly in a time $t$. On the surface of $B$, $2n$ photons of frequency $3f$ are incident perpendicularly in a time $4t$. The ratio of average intensity of radiation on surface $A$ to that on surface $B$ is

The rest energy of an electron is

The energy of a photon of wavelength $\lambda$ is given by

$A$ $2\,mW$ laser operates at a wavelength of $500\,nm.$ The number of photons that will be emitted per second is [Given Planck's constant $h = 6.6 \times 10^{-34}\,Js,$ speed of light $c = 3.0 \times 10^8\,m/s$]

$A$ transmitter of power $10 \text{ kW}$ emits radio waves of wavelength $500 \text{ m}$. The number of photons emitted per second by the transmitter is of the order of: