Assertion : Photoelectric effect demonstrates the wave nature of light.

Reason : The number of photoelectrons is proportional to the frequency of light.

Assertion : $X$-rays travel with the speed of light.

Reason : $X$-rays are electromagnetic rays.

Assertion : Kinetic energy of photo electrons emitted by a photosensitive surface depends upon the inten sity of incident photon.

Reason : The ejection of electrons from metallic surface is possible with frequency of incident photon below the threshold frequency. 

A radioactive nucleus of mass M emits a photon of frequency v and the nucleus recoils. The recoil energy will be

Monochromatic light of wavelength $667 \text{nm}$ is produced by a helium neon laser. The power emitted is $9 \text{mW}$. The number of photons arriving per second on the average at a target irradiated by this beam is