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Speed of light is universally constant so in denser media its collision increases so effective speed will decrease so how we distinguish this effective speed and $3\times10^8$ m/s.
But why sound waves travel fast in denser mediums? collisions are there too but we say that due to elasticity sound travels fast at metals. So why light travels slower and sound travels faster in denser mediums?
As Bill N pointed out in a comment, your claim is not true.
There are two factors that affect the speed of propagation of a mechanical wave. One is some measure of inertia, and the other is some measure of stiffness. The speed of sound is proportional to the square root of the ratio of these quantities. For example, if you compare aluminum with air, the aluminum has a much higher density, which would tend to lower the speed of sound, but aluminum is also much more incompressible than air, which would tend to raise it. The latter effect turns out to be bigger, so the speed of sound in aluminum is higher (about 5000 m/s).
Speed of light is universally constant so in denser media its collision increases so effective speed will decrease so how we distinguish this effective speed and 3×108 m/s.
This isn't something that should be described in terms of collisions. Maxwell's equations describe the speed of light in a medium as depending on its electrical properties: dielectric constant and magnetic permeability.
Sound waves need particles to propagate. They move forward by vibrating particles in medium. So if it is a denser medium sound energy can vibrate more particles in less time since in denser mediums molecules are packed tight. Hence in absence of medium sound cannot travel because no particles exist.
In case of light, it is a electromagnetic radiation. It propagates using electric and magnetic fields. Hence it does not require medium to travel. If it enters medium the molecules in the medium absorb some parts of its energy and reradiate it. This consumes time. Hence its effective speed is decreased.
