A gun recoils , when the bullet is fired from it , why?

When a bullet is fired from a gun, the gun exerts a force on the bullet in the forward direction. This is force is called as the action force. The bullet also exerts an equal and opposite force on the gun in the backward direction. Therefore a gun recoils when a bullet is fired from it.

frequency or wavelenth of em radiation in a vacuum

force tends to be low and long diustance masses are high