Re: sir.Newton said 'light travels with great velocity in denser medium'why?

I've had to do a bit of background reading on this one!  From what I can 
gather, Newton's reasoning must have been the following:

1) light consists of particles so optical effects must be explained by 
mechanical action on these particles.

2) in the phenomenon of refraction, when light moves from a rarer medium 
to a denser one (from air into water, for instance), the rays bend towards 
the normal line; and from step 1 above, this bending must be due to a 
force exerted on the light particles by the denser medium.

3) since in this situation the dense medium is symmetrical in the 
direction parallel to its surface, there can't be any net force in that 
direction, and hence the force must act perpendicular to (and close to) 
the surface layer.  Thus to get the ray to bend you need to have the force 
accelerating the light corpuscles as they reach the surface with the 
result that their speed increases in the dense medium.

4) the nature of the force must be such as to reproduce Snell's law.

So, interestingly, although Newton was antagonistic to the wave theory of 
light (proposed by Huygens), he ended up with a prediction that was 
reminiscent of the properties of sound waves - namely that the wave 
travels faster through a denser (strictly speaking, more rigid) medium.

The physics of sound production was well understood in Newton's time, both 
theoretically and experimentally.  Newton's arch rival, Robert Hooke, for 
instance measured the speed of a fly's wings by comparing the sound it 
made with the sound of a tuning fork!