As the soldiers walk onto the sand they slow down and so
each line of soldiers bends. They end up moving in a different direction. (See Figure 1). The
same thing would happen with an unpowered car or truck rolling from the tarmac onto the
sand.
Now think about a beam of light travelling through air and then hitting a block
of glass at an angle. The light slows down when it enters the glass. It also changes
direction.
When a beam of light hits the glass at an angle the side that hits the
glass first slows down first and so the whole beam slews round, changing direction just like
the group of soldiers. We call this change of direction refraction. You can see this in
Figure 2.
The bigger the change of speed the more the light will
refract. Light moves slower in diamond than it does in glass and so there will be more
refraction when a beam of light moving in air hits a diamond than if it hit a piece of glass at
the same angle.
