Explain what happens to a ray of light as it passes from air into a glass block, and then back out from the glass block into air. Include what happens to the speed and direction of the light at each boundary.

When a ray strikes the air–glass interface it slows down because the refractive index of glass is higher than that of air. The reduction in speed causes the ray to bend towards the normal – the direction perpendicular to the surface.

Inside the glass the ray continues with the lower speed but still travels in a straight line until it reaches the second interface. At the glass–air boundary the ray speeds up again as it enters the less dense medium. The increase in speed makes the ray bend away from the normal.

The emerging ray is therefore parallel to the original incident ray, but it is displaced laterally because of the two refractions.