The de-Broglie wavelength of an electron is the same as that of a photon. If the velocity of the electron is $25 \%$ of the velocity of light,then the ratio of the $K.E.$ of the electron to the $K.E.$ of the photon will be:

Photo cells are used for the:

The particle nature and wave nature of electromagnetic waves and electrons can be demonstrated by which of the following?

$A$ monochromatic light is incident on a metallic plate having work function $\phi$. An electron,emitted normally to the plate from a point $A$ with maximum kinetic energy,enters a constant magnetic field,perpendicular to the initial velocity of electron. The electron passes through a curve and hits back the plate at a point $B$. The distance between $A$ and $B$ is (Given: The magnitude of charge of an electron is $e$ and mass is $m$,$h$ is Planck's constant,and $c$ is velocity of light. Take the magnetic field exists throughout the path of electron).

The spectrum of an oil flame is an example of:

Assertion $(A)$: In the photoelectric effect,on increasing the intensity of light,both the number of electrons emitted and the kinetic energy of each of them increase,but the photoelectric current remains unchanged.
Reason $(R)$: The photoelectric current depends only on the wavelength of light.