Under what law bangaru lakshman is being punished?

It is a political gimmic. Political rivalry is the reason behind.

The sting operation is conducted by a publication which always writes in favour of one particular party and against the rivals of that party. That publication never ventures to expose the party whom that support. In reality, if that magazine starts sting operation on that particular party, the courts in India will be flooded with corruption cases.

But that publication will never do that. Because they have to protect somebody's interest.

when punishment is there , there must be some penal offence.

what are the sections and what are the acts, under which punishment given?

The sections no. were given in the judgememnt.  But still I feel that the Courts are meant for looking into 'real cases' and not 'fake cases'. The Prevention of Corruption act is applicable to real cases. It may be possible, to give sanctity to a string operation, I think, it is a must that a judicial officer should be present officially during the operation.   I dont know in the present case, any Judicial officer was present officially.