How does light reflect off surfaces and bend when it changes speed?
Reflection and the law angle of incidence equals angle of reflection, refraction as a change of speed and direction at a boundary, and total internal reflection.
A CCEA GCSE Physics answer on the law of reflection, how light refracts when it changes speed at a boundary, the direction of bending, and total internal reflection with its uses in optical fibres.
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What this dot point is asking
CCEA wants you to state the law of reflection, describe refraction as a change of speed and direction when light crosses a boundary, predict the direction of bending, and describe total internal reflection and its uses. Diagrams are central, so always draw the normal.
The answer
Reflection
A smooth, shiny surface gives a clear (specular) reflection; a rough surface scatters light (diffuse reflection), which is why most objects do not act as mirrors.
Refraction
Total internal reflection
When light travels from a dense medium to a less dense one, increasing the angle of incidence eventually reaches the critical angle.
Worked example: a ray through a glass block
Examples in context
- Example 1. A straw in a glass of water
- The straw looks bent at the water surface because light from the underwater part refracts as it leaves the water into air, changing direction and making the straw appear displaced.
- Example 2. Broadband through optical fibres
- Data is sent as pulses of light down thin glass fibres. Total internal reflection keeps the light bouncing along inside the fibre with very little loss, carrying signals over long distances at high speed.
- Example 3. A swimming pool looking shallow
- A pool looks shallower than it really is because light from the bottom refracts away from the normal as it leaves the water into air. Your brain assumes the light travelled in a straight line, so the bottom appears raised, a direct everyday effect of refraction.
In the exam, a clear ray diagram earns marks: draw the boundary, add the normal as a dashed line at the point of incidence, and label the incident, reflected and refracted rays with their angles measured from the normal. Showing the ray bending the correct way (towards or away from the normal) is essential.
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 as it enters glass from air, and why? [2 marks]
- Cue. Towards the normal, because it slows down in the denser glass.
Q3. Name one use of total internal reflection. [1 mark]
- Cue. Optical fibres (or prisms in binoculars or periscopes).
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, measured from the normal. State the angle of reflection and the law that applies, and explain what is meant by the normal.Show worked answer →
The angle of reflection equals the angle of incidence, so it is 35 degrees.
The law of reflection states that the angle of incidence equals the angle of reflection (both measured from the normal).
The normal is the line drawn at right angles (perpendicular) to the surface at the point where the ray hits.
Markers reward the angle 35 degrees, the law of reflection, and a correct definition of the normal.
CCEA style4 marksDescribe what happens to a ray of light as it passes from air into a glass block, and explain why it changes direction.Show worked answer →
As the light enters the glass it slows down because glass is optically denser than air.
Because it slows, the ray bends towards the normal as it enters the glass.
On leaving the glass into air it speeds up again and bends away from the normal.
Markers reward: light slows in glass; bends towards the normal on entering; speeds up and bends away from the normal on leaving (the change of speed causes the bending).
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)