Chapter 1 — Light – Reflection and Refraction
Light travels in straight lines — until it meets a mirror or enters glass. Two simple rules, reflection and refraction, explain every mirror, lens, camera and pair of spectacles.
Switch on a torch in a dark room and the beam runs dead straight to the wall. Yet the same light, a moment later, can be turned back by a mirror, squeezed to a dazzling point by the shiny bowl behind the bulb, bent as it slips into a glass of water — making a straight pencil look broken — and focused by the tiny lens of a phone camera into a sharp picture on a sensor smaller than your fingernail. Everything that light does in this chapter comes down to just two behaviours at a surface: it bounces back (reflection) or it goes through and changes direction (refraction).
Engineers use these two rules every day. Car designers choose convex side mirrors so drivers see a wide stretch of road; optometrists prescribe lenses measured in dioptres; scientists at India's observatories collect starlight with huge concave mirrors. In this chapter you learn to draw exactly where the light goes, to predict where an image forms and how big it is — first by ray diagrams, then by a short formula — and to measure the "strength" of a lens. These are also the ideas on which the human eye and the colours of the sky (Chapter 2) depend.
| Everyday device | What the light does | Section |
|---|---|---|
| Bathroom mirror, periscope | reflects from a flat surface, $\angle i=\angle r$ | 1.1 |
| Torch reflector, shaving mirror, rear-view mirror | reflects from a curved surface | 1.1, 1.2 |
| Swimming pool that looks shallow | bends as it leaves water for air | 1.3 |
| Spectacles, magnifying glass, camera | bends twice, passing through a lens | 1.4 |
- state and use the laws of reflection, and describe the image in a plane mirror;
- name the parts of a spherical mirror, use $R=2f$, and draw ray diagrams for concave and convex mirrors for every object position;
- use the New Cartesian sign convention with the mirror formula $\frac1v+\frac1u=\frac1f$ and magnification $m=-\frac vu$;
- state the laws of refraction, use Snell's law and find refractive indices from $n=\frac cv$;
- draw ray diagrams for convex and concave lenses, use the lens formula $\frac1v-\frac1u=\frac1f$, $m=\frac vu$, and find the power of a lens in dioptres.
Sections in this chapter
Reflection of Light and Spherical Mirrors
The laws of reflection, plane-mirror images, concave and convex mirrors, and why the focus sits halfway to the centre.
Image Formation, the Mirror Formula and Magnification
Ray diagrams for every object position, the New Cartesian sign convention, and calculating images.
Refraction of Light and the Refractive Index
Why light bends, the laws of refraction, the glass slab, and Snell's law.
Spherical Lenses, the Lens Formula and Power
Convex and concave lenses, ray diagrams, the lens formula, and lens power in dioptres.