# Question #9c493

Apr 18, 2016

It will not change path if it is incident along the normal

#### Explanation:

When light moves from say air into glass, if its angle of incidence is ${0}^{0}$ (i.e. it is along the path of the normal), then the light will slow down but not change path

Apr 18, 2016

Yes.

#### Explanation:

It depends on what you call the "speed of light".

Being an EM wave, it has two definitions of velocity.

1. Phase velocity :

Given as the velocity with which the phase front of the EM wave moves, This is the regular "velocity" we talk about. In space it is $3 \times {10}^{8} \text{ m/s}$. And in any medium it is decreased by a factor of $\mu$ which is the refractive index of the medium. Now, since in most medium with high $\mu$ absorbs light as well, it has hard to "see" light wave propagating slowly.

2 . Group velocity:

This is how the "shape" of the wave travels.

By Geek3 - Own work;This mathematical image was created with Mathematica, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=14772688

Now depending on how the medium "reacts" to an incident light wave, this velocity can be decreased, and a beam has actually been stopped.

Interesting video: enter link description here

There you go .. it can be "slowed down", a lot .

Thank you for asking this.