Reflection of Waves
This occurs when a wave hits the boundary between two mediums. At it hits the boundary, some of its energy is reflected, for example seeing a reflection in puddles – visible light waves are reflected into your eyes.
Visible light is reflected because of a change in density, as water is denser than air and so whenever a wave reaches a medium with a different density, some of the wave's energy is reflected at the boundary.
Remember: the angle of reflection is the same as the angle of incidence.
Refraction of Waves
Waves also travel at different speeds in mediums which have different densities. Electromagnetic (EM) waves travel slower in dense media, whereas sound waves travel faster in them. Therefore, when a wave crosses a boundary between two mediums, it changes in speed.
An example of refraction is when light passes from air into glass when shining through a window. The glass is a denser medium than air, and so the visible light wave slows down, causing the ray of light to change direction towards the normal.
When this ray of light has passed through the glass and reaches the boundary between glass and air, it is passing into a less dense medium and so speeds up, changing direction away from the normal. Some light is also reflected at this point.
The light ray emerges from the window pane on the other side and is travelling in the same direction as it was when it went in.
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