how choke in tube light gives high voltage impulse at the time of starting and how it gives normal voltage after the tube glown... any one explain please

Hello boby

There is a starter which operates when the voltage across the tube is high, and puts a temporary short across the tube and then releases. This creates a field in the choke which then collapses generating a voltage strike due to the back emf of the collapsing field.
When the tube is lit, the voltage is lower and the starter does not strike.

Best regards,