A photon falls through a height of $1 \,km$ through the earth's gravitational field. To calculate the change in its frequency, take its mass to be $h v / c^{2}$. The fractional change in frequency $v$ is close to

Photo cells are used for the

In photo electric effect

$A.$ The photocurrent is proportional to the intensity of the incident radiation.

$B.$ Maximum Kinetic energy with which photoelectrons are emitted depends on the intensity of incident light.

$C.$ Max. $K.E$ with which photoelectrons are emitted depends on the frequency of incident light.

$D.$ The emission of photoelectrons require a minimum threshold intensity of incident radiation.

$E.$ Max. K.E of the photoelectrons is independent of the frequency of the incident light.

Choose the correct answer from the options given below:

In an accelerator experiment on high-energy collisions of electrons with positrons, a certain event is interpreted as annihilation of an electron-positron pair of total energy $10.2\; BeV$ into two $\gamma$ -rays of equal energy. What is the wavelength associated with each $\gamma$ -ray? $\left(1\; BeV =10^{9}\; eV \right)$

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 $h\, = 6.62\times10^{- 34}\, Js$]

The time taken by a photoelectron to come Out after the photon strikes is approximately