Question

In the rainy season the flash of the lightning is seen first and the sound of thunder had a little later why

Answer 1

Because the sound of thunder only travels at the speed of sound, while the light from the flash travels at the speed of light. The speed of sound is about 340 meters per second through air, while the speed of light is about 300 million meters per second - almost a million times faster. So, if you were 1500 meters away from the flash, you would see it about 5 millionths of a second after it actually occurred, but you wouldn't hear the thunder until about 5 seconds later.