Light of wavelength $5000 \, \mathring{A}$ falls on a sensitive surface. If the surface has received $10^{-7} \, \text{J}$ of energy,then the number of photons falling on the surface will be:

Find the number of photons emitted per second by a $10 \ kW$ radio transmitter operating at a wavelength of $500 \ m$.

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

$A$ photon of wavelength $\lambda$ is scattered from an electron,which was at rest. The wavelength shift $\Delta \lambda$ is three times of $\lambda$ and the angle of scattering $\theta$ is $60^o$. The angle at which the electron recoiled is $\phi$. The value of $\tan \phi$ is: (electron speed is much smaller than the speed of light)

$A$ Laser light of wavelength $660\,nm$ is used to weld Retina detachment. If a Laser pulse of width $60\,ms$ and power $0.5\,kW$ is used,the approximate number of photons in the pulse are: [Take Planck's constant $h = 6.62 \times 10^{-34}\,Js$]

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