Slow medium

• Light travels through a vacuum at 299,792 kilometres per second.
• Light travels through other materials at lower speeds. In some materials, it travels at a speed closer to the speed of light in a vacuum, and in others, it travels much more slowly.
• The speed of light in air is about 299,702 kilometres per second whilst it travels at approximately 123,889 kilometres per second through diamond.
• It is useful to know whether a material is fast or slow to predict what will happen when light crosses the boundary between one material and another.
  • As light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
  • As light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.

Light & Refraction

• The bending of light and accompanying change in speed, when it passes from one medium to another, is called refraction.
• The amount of bending depends on the difference in the speed of light in the two materials and the angle at which the light enters the new material.
• The refractive index of a material is a measure of how much light slows down when it enters that material from a vacuum.