GCSE Physics

Light and optics

WJEC 3420QS — matched to your course.

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Learning goals4

Questions available2

Retrieval cards available1

Available question marks3

Subtopics and learning goals

Apply the laws of reflection and explain refraction using changing wave speed.
Apply the law of reflection.
Explain refraction in terms of a change of wave speed.
Relate changes of speed and wavelength while frequency remains constant.

Revision notes

Refraction occurs when light changes speed on entering a different medium. If light enters a medium in which it travels more slowly at an angle to the normal, it bends towards the normal; when it speeds up it bends away from the normal. The frequency is fixed by the source and remains unchanged at the boundary, so a change of speed is accompanied by a change of wavelength. At normal incidence the direction does not change even though speed and wavelength may.

Reflection and ray diagrams

Ray diagrams should use straight rays, arrowheads for direction and a normal where angles are measured. Accuracy matters: examiners use the geometry to judge understanding.

Refraction

Refraction occurs when light changes speed at a boundary. Frequency stays constant because it is set by the source, while wavelength changes with speed. When entering a medium where light travels more slowly, a ray usually bends towards the normal; when speeding up, it bends away from the normal.

At normal incidence the speed and wavelength may change but the direction does not. This is a useful reminder that refraction is fundamentally about changing speed, not simply changing direction.

Total internal reflection

Total internal reflection occurs when light travels from a higher refractive-index medium towards a lower refractive-index medium and the angle of incidence exceeds the critical angle. It is used in optical fibres and reflecting prisms.

Worked example

Worked example 1: A ray incident on a plane reflector at 35° to the normal is reflected at 35° to the normal.

Worked example 2: A wave slows from 300 m/s to 225 m/s at constant frequency 150 Hz. Its wavelength changes from 2.0 m to 1.5 m.

Misconception check

When light refracts, its frequency does not change at the boundary. Its speed and wavelength change together.

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2 questions available, worth 3 marks in total.

Calculation 1Explanation 1

Foundation 2

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