Refraction, Why does the light re accelerate?

[josh444]

OK, so i think i get why refraction happens and it makes sense that as the light passes into something of a higher refractive index it will decelerate and if it hits at an angle, one "edge" of the beam will hit first and thus descelerate before the other side of the beam causing the beam to bend.

However what i don't understand is why as the beam exits the , let's say block of perspex, and re enters air , the light increases its velocity again causing it to bend. Now I am not contesting the fact that it accelerating would cause it to bend but I am just struggling to understand what is causing the beam to accelerate, as i was under the impresion that upon descelerating it lost some of its kinetic energy (i gues to the block of perspex as heat maybe?) and thefore inorder to accelerate out the other side it would have to regain this kinnetic energy from somewhere and i just don't get where this extra energy comes from.

any one know?

thanks
josh

 

[ZapperZ]

You may want to read our FAQ in this section of PF to get some idea on the nature of optical transport in matter.

Zz.

 

[Andrew Mason]

You may want to read our FAQ in this section of PF to get some idea on the nature of optical transport in matter.

A most insightful explanation - it is worth a direct link https://www.physicsforums.com/showthread.php?t=104715#4".

AM