Question

When light travels form rare to dense medium what happens to its speed\?

Answer 1

reduces but remember that density mentioned here is optical density

Answer 2

A ray of light whem passed from dense medium to rare medium, changes its path.

It changes its path as by bending itself away from the normal ray.

This happens because of difference in density, which allows the change in speed of light as well as direction during refraction.

So, we can say that light passing from dense medium to rare medium deviates its path and bends away from normal, and experience increase in speed.