Why does light change direction when entering a new medium?

[jnorman]

a wave will be diffracted as it passes by an object. in the case of light passing through a slit experiment, why isn't the probablility function of the associated photon(s) "collapsed" by its interaction with the edge of the slit?

a wave will be refracted if it enters a new medium. what is it about entering a new medium that would cause a change in direction of the wave? why is the directional change always the same with a given angle of incidence? why isn't it random?

i have actually read a lot of physics during my years as a sentient being, and it still seems like i truly understand nothing at all... things can seem so simple on the surface, but as you drill down into what is actually happening, you eventually get to a place where it is beyond comprehension.

 

[cragar]

When light goes into a medium the electric and magnetic components of the wave are affected by the electric and magnetic constants of the medium so that's why it changes its path. And maybe the reason that it isn't random is that light has momentum.
You should watch the Feynman lectures on light.
http://www.vega.org.uk/video/subseries/8
he talks about why light hitting a mirror will come off at the angle it hits. And why a lens bends light .