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If an optically denser medium need not have greater mass density then why speed is reduced in that medium?? What causes the speed to slow down?? Jaldi batao!!

Dear Student,

The refraction of light takes place on going from one medium to another because the speed of light is different in two media. Thus, when light goes from one medium to another, its speed changes. This change in speed of light causes refraction of light. The ability of a material to refract light is expressed in terms of optical density. A material with higher refractive index is said to be optically denser than that having a lower refractive index. You should not confuse optical density with mass density of a material. The substance may have higher optical density but lower mass density or vice versa. 

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an optically denser medium has its own refractive index which will not be same as the first medium hence the speed of light in that medium is reduced.
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Speed of light in a medium is based on its optical density and not its mass density. 
When light travels from an optically rarer medium to an optically denser medium, it bends towards the normal, and so, to maintain the time of travel, the speed of light is reduced. 
The mass density does not affect this phenomenon.

Hope this helps!
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