Light
Reflection in a plane mirror, refraction and refractive index, total internal reflection, thin converging lenses and dispersion.
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Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 3.2.1Describe reflection at a plane surface and construct ray diagrams for an image in a plane mirror.
- 3.2.2Describe refraction at a boundary and recall and use n = sin i / sin r.
- 3.2.3Explain total internal reflection and the critical angle, including uses in optical fibres.
- 3.2.4Draw ray diagrams for a thin converging lens forming real and virtual images.
9 minute read
Reflection, refraction and lenses
Reflection in a plane mirror
The law of reflection: the angle of incidence equals the angle of reflection, both measured from the normal. The image in a plane mirror is always: virtual (light does not actually pass through it), upright, the same size as the object, laterally inverted (left-right reversed), and as far behind the mirror as the object is in front.
Refraction
Light bends when it crosses a boundary between materials of different density, because its speed changes. Going into a denser material, light slows down and bends towards the normal; leaving a denser material, it speeds up and bends away from the normal.
The refractive index is n = sin i / sin r.
Total internal reflection
Above the critical angle, light travelling from a denser into a less dense medium cannot refract out at all — it reflects entirely back inside. This is total internal reflection, and it is the principle behind optical fibres, which carry light (and digital signals) over long distances by repeated total internal reflection along the inside of a thin glass fibre.
Converging lenses
A thin converging lens brings parallel rays of light together at the principal focus. Depending on where the object is placed relative to the focal length, the lens forms either a real, inverted image (object beyond the focal length — as in a camera) or a magnified, virtual, upright image (object inside the focal length — as in a magnifying glass).
Think of it like this
Refraction is like a marching band crossing from pavement onto sand at an angle — the row of marchers who reach the sand first slow down, so the whole line pivots and changes direction, exactly as a wavefront bends when it enters a new medium.
Worked examples
Method, step by step
Light travels from glass (n = 1.5) into air. Calculate the critical angle.
- 1At the critical angle, the angle of refraction is 90°, so sin r = 1.
- 2n = sin i / sin r, so sin(critical angle) = 1 / n
- 3sin c = 1 / 1.5 = 0.667
c = 41.8°
Common misconceptions
- Measuring angles of incidence and reflection from the mirror surface instead of the normal — always measure from the dashed line at 90° to the surface.
- Thinking a virtual image can be projected onto a screen. It cannot — light rays only appear to come from it; a real image can be captured on a screen.
- Confusing total internal reflection with ordinary reflection — total internal reflection only happens above the critical angle, and only when light tries to leave a denser medium.
In the exam
- For plane mirror ray diagrams, use the "same distance behind as the object is in front" rule to place the image before drawing the reflected rays.
- When describing an image, always give all relevant properties: real/virtual, upright/inverted, magnified/same size/diminished.