What happens to the ray of light each time it strikes the boundary between glass and air in an optical fibre?

Answer 1

It slows down.
In regular circumstances, the speed of light (c^2) is the same. Photons, or light particles act as both a wave and a particle, (wave-particle duality), so this can help to explain it. Think of the light as a stream traveling in a straight line through air, when it goes through glass, the speed is the same but instead of it going straight, it bounces around. Instead of going simply from one side to the other, it has to bounce back and forth between different particles, so it takes longer for it to reach the other side. Think of going to work and taking a shortcut, it's more direct to the place you want to go, but now there's a road block and you have to go the long way. You can be travelling at 40 km/hr in both situations, it just takes longer because you're taking a longer route and travelling a longer distance to get to the same place.
Science does not yet understand what exactly happens when light goes through glass. There are plenty of theories, but a serious scientist that specializes in this area will admit that it is not known. ------------------------------ ANSWER: Basically, as light hits a block of glass, passing from air, it refracts, or bends towards the "normal", which is an imaginary line which sits at right angles to the line of glass that the light went into. When it leaves the block of glass, then it refracts once more, away from the "normal".
Since the light is coming from a more less dense environment (air) to a more dense object (glass) the light will tend to bend towards the normal. The normal is an imaginary line which is perpendicular to the surface of the object. This process is called Refraction and Snells Law can be applied to do stuff involving refraction.
The light slows down and refracts.

Answer 2

The ray refracts back into the fibre.

On the most efficient optical fibres the ray of light will never come near the boundary between glass and air. These fibres have a glass core and an outer glass jacket around that with a different index of refraction. The ray refracts back into the core at the boundary between the core and the jacket.

Answer 3

The beam of light will change its direction - depending on the angle of incidence.