Explain how waves will be refracted at a boundary in terms of the change…

The idea

Refraction happens when a wave crosses a boundary between two materials and its speed changes. If the wave arrives at an angle (not along the normal), the change in speed bends its direction. Going into a denser medium the wave slows down and bends towards the normal; going into a less dense medium it speeds up and bends away from the normal. A wave travelling along the normal changes speed but does not change direction. The frequency stays the same across the boundary; the wavelength changes because speed changes.

For example: Light passing from air into glass slows and bends towards the normal, and on leaving glass into air it speeds up and bends away.

What earns the marks

• States that the speed of the wave changes when it crosses the boundary between two materials.
• Explains that a change of speed at an angle causes the wave to change direction.
• Identifies that a wave slowing down bends towards the normal.
• Identifies that a wave speeding up bends away from the normal.
• Recognises that a wave travelling along the normal changes speed without changing direction.
• States that frequency is unchanged at the boundary while wavelength changes with speed.

Common mistakes and how to put them right

• Claims the wave bends because its frequency changes.

  Put it right by stating frequency stays constant and the speed and wavelength change.

• Says a wave bends towards the normal when it speeds up.

  Put it right by linking slowing down to bending towards the normal.

• Assumes a wave always changes direction at a boundary.

  Put it right by noting a wave along the normal changes speed but not direction.

• Confuses reflection with refraction.

  Put it right by describing refraction as a change of speed and direction on crossing the boundary.

Exam tips

• Link each direction change to whether the wave speeds up or slows down, naming the normal explicitly.
• Use the phrase 'towards the normal' or 'away from the normal' rather than vague wording such as 'bends more'.
• Remember that frequency is constant at a boundary, so any change in speed must be matched by a change in wavelength.