Why does light bend when it passes from one material into another?
Refraction of light: the change of direction when light crosses a boundary between materials, the angles of incidence and refraction, why refraction happens, and the action of a lens and total internal reflection.
An SQA National 5 Physics answer on the refraction of light, covering the change of direction when light crosses a boundary, the angles of incidence and refraction measured from the normal, why refraction happens (a change of speed), how a lens uses refraction, and total internal reflection in optical fibres.
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What this key area is asking
The SQA wants you to describe how light changes direction when it crosses a boundary, label the angles of incidence and refraction from the normal, explain why refraction happens, and describe the action of a lens and total internal reflection.
What refraction is
The normal is an imaginary line drawn at right angles to the surface at the point where the ray hits. Always measure the angles from this line, because measuring from the surface gives the wrong angle.
Why light bends
This change of speed is why a straw in a glass of water looks bent and why a swimming pool looks shallower than it is. The frequency of the light stays the same, but its speed and wavelength change in the new material.
Lenses
A lens works by refraction. A convex (converging) lens is thicker in the middle; it refracts parallel rays so that they meet at a focal point, and is used in magnifying glasses, cameras and to correct long sight. A concave (diverging) lens is thinner in the middle and spreads rays out. The action of the lens comes entirely from light bending as it enters and leaves the curved glass surfaces.
Total internal reflection
When light inside a denser material meets the boundary at an angle greater than a particular critical angle, none of it escapes; instead it is all reflected back inside. This is total internal reflection. It is used in optical fibres, where light is repeatedly reflected along the inside of a thin glass fibre, carrying telephone, internet and TV signals over long distances with very little loss. It is also used in reflecting prisms in binoculars and periscopes.
Try this
Q1. State from which line the angles of incidence and refraction are measured. [1 mark]
- Cue. The normal (a line at right angles to the surface).
Q2. A ray passes from air into glass. State whether it bends towards or away from the normal, and why. [2 marks]
- Cue. Towards the normal, because it slows down entering the denser glass.
Q3. Name one use of total internal reflection. [1 mark]
- Cue. Optical fibres (or reflecting prisms in binoculars/periscopes).
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksA ray of light travels from air into a glass block. State how the angle of refraction compares with the angle of incidence, and explain why the ray changes direction.Show worked answer →
When light passes from air into glass (a denser material), the angle of refraction is smaller than the angle of incidence, so the ray bends towards the normal.
This happens because the light slows down as it enters the glass. The change in speed at the boundary causes the change in direction.
Markers reward stating that the ray bends towards the normal (angle of refraction less than angle of incidence) and linking the bending to the change in speed of the light.
SQA N5 style3 marksExplain how total internal reflection allows light to travel along an optical fibre.Show worked answer →
Light travelling inside the fibre strikes the inside surface at an angle greater than the critical angle for the glass.
When the angle of incidence inside the denser material is greater than the critical angle, all the light is reflected back inside instead of escaping. This is total internal reflection.
So the light is repeatedly reflected along the inside of the fibre and carried from one end to the other with very little loss. Markers reward the idea of the angle exceeding the critical angle and total internal reflection keeping the light inside the fibre.
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Sources & how we know this
- SQA National 5 Physics Course Specification — SQA (2019)