Light and force We know that photons also have momentum though they do not have mass given by the equation :
$E=pc$
Then why can't we apply force to light? I mean we can apply fore and increase its momentum and thus its energy.
Also if light collides with something (which is I am not sure) than will it follow the law of conservation of momentum?
 A: You can apply a force to light. Simply shine the light onto a mirror then move the mirror towards the incoming light:

The light that reflects off the mirror will be blue shifted to a shorter, higher energy wavelength, $\lambda_\text{out} \lt \lambda_\text{in}$, and you'll need to apply a force to the mirror to achieve this blue shift.
A: We can apply force to light:  you can act on light with gravitation. Even though light does not have electric charge, it has some (small) sensitiveness to electromagnetic fields. This comes from loop diagrams in quantum electrodynamics. So you can act on light by EM fields.
"will it follow the law of conservation of momentum?"
Of course.
Light does not have "rest mass". It has relativistic mass due to its speed. This mass has all properties of mass. Although sometimes physicists have philosophical arguments about nature of relativistic mass, by its properties the relativistic mass is a "well behaved true" mass. So if you want light has a mass: a stopped photon does not exist, an existing photon moves at with $c$ velocity and has (relativistic) mass.
