If electromagnetic waves of power 600 W incident on a non-reflecting surface, then the total force acting on the surface is

When a photosensitive material is illuminated by photons of energy 3.1 eV , the stopping potential of the photoelectrons is 1.7 V . When the same photosensitive material is illuminated by photons of energy 2.5 eV , the stopping potential of the photoelectrons is

The ratio of the kinetic energies of the electrons in the third and fourth excited states of hydrogen atom is

In $\beta^{-}$decay, a neutron transforms into a proton within the nucleus according to the equation :

neutron $\rightarrow$ proton $+\beta^{-}+x$

In this equation the particle represented by ' $x$ ' is