A photon in motion has a mass

A $160 \,W$ infrared source is radiating light of wavelength $50000 \,\mathring A$ uniformly in all directions. The photon flux at a distance of $1.8 \,m$ is the order of …………. $m^{-2} s ^{-1}$

When monochromatic radiation of intensity $I$ falls on a metal surface, the number of photoelectrons and their maximum kinetic energy are $N$ and $K$ respectively. If the intensity of radiation is $2I$, the number of emitted electrons and their maximum kinetic energy are respectively

The light of two different frequencies whose photons have energies $3.8 \,eV$ and $1.4 \,eV$ respectively, illuminate a metallic surface whose work function is $0.6 \,eV$ successively. The ratio of maximum speeds of emitted electrons for the two frequencies respectivly will be

A sensor is exposed for time $t$ to a lamp of power $P$ placed at a distance $l$. The sensor has a circular opening that is $4d$ in diameter. Assuming all energy of the lamp is given off as light, the number of photons entering the sensor if the wavelength of light is $\lambda $ is $(l >> d)$