A $200\, W$ sodium street lamp emits yellow light of wavelength $0.6\, \mu \,m$. Assuming it to be $50\%$ efficient in converting electrical energy to light, the number of photon of yellow light it emits per second is
$62 \times 10^{20}$
$3 \times 10^{20}$
$1.5 \times 10^{20}$
$6 \times 10^{18}$
If the energy of a photon is $10\,eV$, then its momentum is
A particle which has zero rest mass and non-zero energy and momentum must travel with a speed
A photon of wavelength $6630 \ Å$ is incident on a totally reflecting surface. The momentum delivered by the photon is equal to
Rest mass energy of an electron is $0.51\ MeV.$ If this electron is moving with a velocity $0.8\ c$ (where $c$ is velocity of light in vacuum), then kinetic energy of the electron should be. ........... $MeV$
An electron and proton are separated by a large distance. The electron starts approaching the proton with energy $3\, {eV}$. The proton captures the electrons and forms a hydrogen atom in second excited state. The resulting photon is incident on a photosensitive metal of threshold wavelength $4000\, \mathring {{A}}$. What is the maximum kinetic energy of the emitted photoelectron ? (In ${eV}$)