I am not able to understand reaction of magnetic force and how it is equal to qvb ?
When you start looking at fields you have to generalize Newton's third law. Basically Newtons third law expresses the conservation of momentum. While it may not make a lot of sense to talk about the force on a field, you can certainly talk about its momentum. Any force that the field exerts on matter causes an equal and opposite change in the momentum of the field.