How do waves reflect and refract, and why does refraction happen?
The reflection of waves and the law of reflection, and the refraction of waves as they change speed when crossing a boundary.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on reflection and refraction, covering the law of reflection, how images form in a mirror, and why waves refract when they change speed crossing a boundary.
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What this dot point is asking
WJEC Double Award Unit 3 wants you to describe the reflection of waves and the law of reflection, and explain the refraction of waves as they change speed crossing a boundary.
Reflection
Images in a mirror
A flat (plane) mirror forms an image that is:
- the same size as the object,
- upright (the same way up),
- as far behind the mirror as the object is in front,
- virtual (it cannot be projected onto a screen) and laterally inverted (left and right swapped).
Refraction
When light passes into a denser material (such as glass), it slows down and bends towards the normal. When it passes back into a less dense material (such as air), it speeds up and bends away from the normal. If the light hits the boundary along the normal (at 90 degrees to the surface), it changes speed but does not change direction.
Why the change in speed bends the ray
Think of the wave as a row of marchers crossing from a road onto mud at an angle. The marchers reach the mud one at a time and slow down, so the whole row swings round to a new direction. In the same way, one side of a light wave slows before the other when it enters glass at an angle, so the wave changes direction. This is why refraction only bends the ray when it meets the boundary at an angle.
Wavelength and frequency in refraction
When a wave refracts, its speed and wavelength change but its frequency stays the same. As light slows down entering glass, its wavelength gets shorter, but the same number of waves still pass each second, so the frequency is unchanged. This makes sense because the frequency is set by the source making the wave, not by the material it travels through. Remembering that frequency stays constant while speed and wavelength change is a common higher-mark point in refraction questions.
Everyday examples of refraction
Refraction explains several everyday effects. A straw in a glass of water looks bent at the surface, because light from the underwater part is refracted as it leaves the water. A swimming pool looks shallower than it really is, for the same reason. Lenses in glasses, cameras and the eye work by refracting light to form an image, and a glass prism splits white light into a spectrum because each colour refracts by a slightly different amount. Being able to explain a familiar example using the change in speed at a boundary shows real understanding.
Try this
Q1. From which line are the angles of incidence and reflection measured? [1 mark]
- Cue. The normal.
Q2. Which way does light bend when it enters glass from air? [1 mark]
- Cue. Towards the normal (because it slows down).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style3 marksState the law of reflection and define the angle of incidence.Show worked answer →
A Unit 3 recall question. The law of reflection states that the angle of incidence equals the angle of reflection (1). The angle of incidence is the angle between the incoming ray and the normal (the line at right angles to the surface) (1); both angles are measured from the normal, not the surface (1). Markers credit the law, and the angle measured from the normal. A common error is to measure the angles from the mirror surface instead of the normal.
WJEC style4 marksExplain why a ray of light bends when it passes from air into glass.Show worked answer →
A Unit 3 explain question worth 4 marks. Reward: light travels slower in glass than in air (1); as it crosses the boundary into the denser glass it slows down (1); this change in speed makes the ray change direction (refract), bending towards the normal (1); when it leaves the glass into air it speeds up and bends away from the normal (1). Markers credit the change in speed, bending towards the normal entering glass and away when leaving. A common error is to say the light bends without mentioning the change in speed.
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