How does light reflect and refract, and how do lenses form images?
The law of reflection, refraction as light changes speed and direction at a boundary, total internal reflection, and how converging and diverging lenses refract light.
A CCEA GCSE Double Award Science (Physics Unit P2) answer on the law of reflection, refraction as light changes speed at a boundary, total internal reflection and its uses, and how converging and diverging lenses refract light.
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What this dot point is asking
CCEA Double Award wants you to state the law of reflection, explain refraction as light changing speed at a boundary, describe total internal reflection and its uses, and explain how converging and diverging lenses refract light. Ray diagrams and the angle rules are the heart of these questions.
Reflection
A plane mirror forms an image that is upright, the same size, and virtual, appearing as far behind the mirror as the object is in front.
Refraction
If the ray hits the boundary along the normal (angle of incidence zero), it does not bend, although it still changes speed.
Total internal reflection
Lenses
Examples in context
- Example 1. A straw in water
- A straw in a glass of water looks bent at the surface because light from the underwater part refracts as it leaves the water, so the straw appears displaced.
- Example 2. Optical fibres
- Broadband and telephone signals travel as pulses of light down glass fibres, kept inside by total internal reflection at every bounce, allowing fast data over long distances with little loss.
- Example 3. A magnifying glass
- A converging lens held close to an object produces a magnified, upright virtual image, which is how a magnifying glass enlarges small print.
Try this
Q1. State the law of reflection. [1 mark]
- Cue. The angle of incidence equals the angle of reflection (measured from the normal).
Q2. Which way does light bend when it enters glass from air? [1 mark]
- Cue. Towards the normal (because it slows down).
Q3. Name one use of total internal reflection. [1 mark]
- Cue. Optical fibres (or prisms in periscopes/binoculars).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style3 marksA ray of light hits a plane mirror at an angle of incidence of 35 degrees. State the law of reflection and the angle of reflection.Show worked answer →
The law of reflection states that the angle of incidence equals the angle of reflection (both measured from the normal).
So the angle of reflection is 35 degrees.
Markers reward angle of incidence equals angle of reflection (measured from the normal), and the value 35 degrees.
CCEA-style4 marksExplain what happens to a ray of light as it passes from air into glass, in terms of speed and direction.Show worked answer →
Light slows down when it enters the denser glass.
Because it slows down, it changes direction, bending towards the normal.
(On leaving the glass back into air it speeds up and bends away from the normal.)
Markers reward light slowing in the glass, bending towards the normal on entering, and speeding up or bending away on leaving.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)