AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
(HT only) Some effects, for example refraction, are due to the difference…
Full specification wording
Properties of electromagnetic waves 1: (HT only) Some effects, for example refraction, are due to the difference in velocity of the waves in different substances.
This is Higher tier.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This is Higher tier. Refraction is a change of direction caused by a change of speed. An electromagnetic wave travels fastest in a vacuum and slows down in a material, and the more optically dense the material the slower it goes, so light travels more slowly in glass than in air. When a ray meets a boundary at an angle, one side of the wave enters the new material and slows before the other side does, so the whole wave swings round. Going from air into glass the wave slows and bends towards the normal; going from glass into air it speeds up and bends away from the normal. A ray arriving along the normal, at an angle of incidence of 0°, still changes speed but does not change direction, because both sides slow together.
What earns the marks
- Stating that the wave changes speed when it enters a different substance.
- Saying it bends towards the normal when it slows down and away from the normal when it speeds up.
- Explaining that the change of direction is caused by the change of speed, not by the boundary itself.
- Saying that a ray travelling along the normal changes speed but not direction.
Common mistakes and how to put them right
Saying light bends because the glass pushes it, with no mention of a change of speed.
Claiming light speeds up in glass because glass is transparent.
Bending the ray the wrong way as it enters a denser medium.
Saying the frequency changes on refraction, when it is the speed and the wavelength that change.
Exam tips
- Always name the direction relative to the normal: towards it or away from it.
- Say which medium is the faster and which the slower before you state which way the ray bends.
- Remember the frequency is set by the source and does not change at the boundary.
Try the lesson
The start of the Properties of electromagnetic waves 1: (HT only) Some effects, for example refraction, are due to the difference in velocity of the waves in different substances. lesson, made from checked teaching notes for this specification point.
Have a go first
Light travels at a different speed in glass from its speed in air. Which effect is caused by this difference in velocity?
Listen
This is Higher tier. Refraction is a change of direction caused by a change of speed. An electromagnetic wave travels fastest in a vacuum and slows down in a material, and the more optically dense the material the slower it goes, so light travels more slowly in glass than in air. When a ray meets a boundary at an angle, one side of the wave enters the new material and slows before the other side does, so the whole wave swings round. Going from air into glass the wave slows and bends towards the normal; going from glass into air it speeds up and bends away from the normal. A ray arriving along the normal, at an angle of incidence of 0°, still changes speed but does not change direction, because both sides slow together. A strong answer earns marks for these points. stating that the wave changes speed when it enters a different substance. saying it bends towards the normal when it slows down and away from the normal when it speeds up. explaining that the change of direction is caused by the change of speed, not by the boundary itself. saying that a ray travelling along the normal changes speed but not direction. Exam tip: Always name the direction relative to the normal: towards it or away from it.
Key words
Say what each word means before you turn the card.
refraction
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change of speed
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towards the normal
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optical density
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See every definition
- refraction
- A change of direction caused by a change of speed when a wave enters a different substance.
- change of speed
- What happens when a wave enters a different substance, and the cause of the change of direction.
- towards the normal
- The way a wave bends when it slows down, for example going from air into glass.
- optical density
- The more optically dense the material, the slower the wave goes in it.
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