Two questions about light

Question

1. I read that light always travels at a constant speed in vacuum, but slows down in transparent mediums. My question is, does light change speeds instantaneously while moving from one medium to another or does it happen gradually?

If it is gradual, then what exactly happens when light emerges into vacuum from a slower medium? Is it possible that while accelerating to speed C, light travels, for a brief moment of time, slower than C in vacuum?

Or if the speed change is instantaneous, what physical process or interaction between light and the particles that make up the medium causes this change?

2. If I am at rest relative to the earth, is it right to say that with respect to an approaching beam of light, I am traveling at speed C? In other words, would a person riding the beam of light see me approaching at C?

Answer 1

1. Sorry, no idea what I was thinking. Upon further thought, the change is referred to as the refractive index. the equation for calculation is m=c/v where

m=refractive index

c=speed of light in a vacuum

v=speed of light in medium

The change is simply refraction so no speed change really, it is caused by minor transparency defects (or atomic colllision really) It is the same as finding out how much light bends relative to speed, also directly relative to the refraction index.

2. If it were possible, yes. Just as if you were on the ground and watching a car approaching, or in the car watching a person either would see the apparent speed of the other, which is only true with respect to the other object because we all know the figure out of the car is stationary, not actually traveling at the speed.

Answer 2

1. I read that light always travels at a constant speed in vacuum, but slows down in transparent mediums. My question is, does light change speeds instantaneously while moving from one medium to another or does it happen gradually?

instantaneously

If it is gradual, then what exactly happens when light emerges into vacuum from a slower medium? Is it possible that while accelerating to speed C, light travels, for a brief moment of time, slower than C in vacuum?

no

Or if the speed change is instantaneous, what physical process or interaction between light and the particles that make up the medium causes this change?

Refraction is the change in direction of a wave due to a change in its speed. This is most commonly seen when a wave passes from one medium to another. Refraction of light is the most commonly seen example, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth.

In optics, refraction occurs when light waves travel from a medium with a given refractive index to a medium with another. At the boundary between the media, the wave's phase velocity is altered, it changes direction, and its wavelength increases or decreases but its frequency remains constant. For example, a light ray will refract as it enters and leaves glass; understanding of this concept led to the invention of lenses and the refracting telescope

2. If I am at rest relative to the earth, is it right to say that with respect to an approaching beam of light, I am traveling at speed C? In other words, would a person riding the beam of light see me approaching at C?

A person cannot travel at the speed of light, so the question is meaningless. But someone approaching you at 99% of the speed of light, would see you approaching at close to the speed of light.

Answer 3

1)  If you go to the microscopic level, light always travels at c.  In a medium, what happens is that the light bops along and keeps getting absorbed and re-emitted by charged particles in the medium that can move, so the disturbance you call light in the medium is a mixture of actual light and moving charged particles that propagates slower than c.  But photons always travel at c.  Always.

2)  Yes, your speed is c relative to the beam of light.  However, nothing with mass can travel at the speed of light, so it doesn't make sense to talk about a person riding the beam of light.