Why does refraction take place? Why does light slow down in denser media?

Question

I read that optically dense medium bends light rays towards the normal. Why does this happen? And why not away from the normal? Why does light slow down in optically dense media?

Please answer only if you can explain the phenomenon well. Otherwise don't bother answering. I know some basics - I want a deeper idea about why it actually happens.

Answer 1

To translate the previous answer into something useful and understandable (in english)...

Light does not pass directly through an object. instead as photons (the energy packets that make up light) strike atoms within the medium, the atoms absorb the photons, causing the electrons within the medium to be temporariliy excited to a higher energy level. The electron then "falls" back to it's normal level, releasing the light from the atom to travel to the next particle. All of this happens very quickly, but over distance can measurably slow the light as it travels through the media.

As to why light bends towards the normal line, it's just like why a car pulls hard to the right if the passenger tires leave the paved road and hit gravel. As the leading edge (let's say the right side) of the light hits the denser media (like a tire hitting the gravel) it slows (just like gravel increases the backwards "drag" on a tire, slowing it). As it slows, this pulls the light to the right (just like the slowing of the right tire pulls the entire car to the right). In other words, the backwards pull on the leading edge of the light pulls the whole ray even harder into the transmission media, in effect bending the beam towards the normal line.

Answer 2

Refraction. or the bending of light, depends upon optical density, or index of refraction, of a substance. "Denser" media are actually defined as media in which light waves propagate more slowly as compared to other media, with a vacuum having zero density. There are media with "negative optical density" in which the rays bend away from the normal.

Light, or more properly the phase velocity of light, is what slows in refractive media. An electromagnetic wave's phase velocity is slowed in a material because the electric field creates a disturbance in the electron charges of each atom. The charges will then oscillate slightly out of phase with respect to the driving electric field. So, such charges radiate their own electromagnetic wave that is at the same frequency except with a phase delay. The aggregate of all such contributions is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase velocity.