a light wave doesnt need a medium to travel
but a sound wave does,why?

Positive and negative electric charges create a field around them. The field extends towards infinity weakening all the time. Gravity field, caused by mass, behaves similarly.

If the charge or mass changes suddenly, the field must change, too, but because the lightspeed is finite, the change is increasingly delayed over distance. This delay makes the change behave as a wave.

If we look at the question "How can a light wave propagate without a medium", it can be changed by the above viewpoint to be "How can electricity and gravity have an effect without a medium"? This is such an advanced question that I cannot answer it, and I do not even know anybody who could.

We may need to learn more about vacuum, because it is obvious that vacuum is not really so empty as it seems to be - there are virtual particles and geometric fluctuations going on in what seems to be a perfect emptiness to us, and the quantum fields of particles extend into the emptiness surrounding them. Vacuum even has interesting properties like energy density and permittivity. A good starting point might be to find out how electromagnetic field [whose changes we see as light] exists and expresses itself in vacuum. Perhaps vacuum can be a kind of medium, too. You may be interested in reading the English Wikipedia entry "Vacuum state".

The propagation of a sound wave is also partly mediumless. Sound waves propagate with air molecules pushing each other, causing pressure maximums transmitted to the adjacent molecules. But the molecules do not touch each other in the same sense as we see objects touching each other. Instead they push each other with an electromagnetic force that increases when the molecules approach each other. This electromagnetic force causing the push is again like a wave, transmitting the push from a molecule to a molecule at lightspeed, without a medium between the molecules. Because the molecules have to move to create a new push somewhere else, the resulting sound wave is slower than lightspeed.