In common base configuration,we know that alpha is almost 1 which means that the base current is almost zero.But what is the physics behind this base current almost equal to 0?And similarly in common emitter the base current is more than that in common base configuration,but why?I mean i want to know the underlying physics behind these concepts.
The physics behind the operation of a bipolar transistor is pretty complicated, so I won't even attempt to give you a detailed explanation, but here is a possible, simplified, way to think about it.
In either configuration, common emitter or common base, the base current is a small fraction of the emitter or collector current.
The base-emitter junction is forward biased, so the current can flow between the emitter and the base. The base-collector junction is reverse biased, so the current cannot flow between the collector and the base.
The reason the current flows between the emitter and the collector is because the collector intercepts charges passing from the emitter to the base.
Let's consider a simplified example of an npn transistor.
When a sufficient forward voltage is applied to the base-emitter junction, the conduction band electrons will flow from the emitter (n-type) to the base (p-type), but as they proceed toward the base terminal, they get into a strong electric field across the reverse biased base-collector junction and get pulled toward the collector.
On their way, some electrons will recombine with the holes of the base and won't be able to get to the collector: they will make it all the way to the base terminal, contributing to the base current. In order to reduce the percentage of electrons recombining with the holes (or in order to increase $\beta$, the ratio between the collector and base currents), the base of the transistor is made very narrow, so that the electrons flowing toward the base terminal are getting very close to the base-collector junction and are likely to be swept away by the base-collector field before they recombine with holes.
In summary, high $\beta$ is achieved due to a special geometry of the base (as well as appropriate doping levels), designed to minimize the recombination rate in the base region and thus to maximize the percentage of electrons crossing the base-collector junction and contributing to the collector current.