Light travels in straight lines until something bends it or bounces it. Mirrors bounce; lenses bend. Everything else in this chapter follows from that.
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When light strikes a surface and bounces back, that is reflection. A smooth surface gives a clear image; a rough one scatters the light.
🪞 The law of reflection
The angle of incidence equals the angle of reflection — always.
Both angles are measured from the normal, the line at right angles to the surface.
The incident ray, the reflected ray and the normal all lie in one plane.
🪞 The three mirrors
| Mirror | Image | Used in |
|---|---|---|
| Plane | same size, upright, laterally inverted, behind the mirror | bathroom mirrors |
| Concave | can magnify; image varies with distance | shaving mirrors, torches, dentists |
| Convex | always smaller and upright, wide view | vehicle rear-view mirrors |
Why vehicles use a convex mirror. It always gives an upright, smaller image and covers a much wider area, so the driver sees more of the road. That is also why such mirrors carry the warning that objects are closer than they appear.
Lateral inversion is why AMBULANCE is painted reversed on the front of the van — so a driver looking in the mirror reads it the right way round.
Light bends when it passes from one medium into another. That bending is refraction.
🔍 The two lenses
| Lens | Shape | Does what | Used in |
|---|---|---|---|
| Convex | thicker in the middle | converges light to a point | magnifying glass, camera, the eye |
| Concave | thinner in the middle | diverges light | spectacles for short sight |
Everyday refraction: a pencil in a glass of water looks bent, a coin in a bowl seems to rise, and a swimming pool looks shallower than it is. All three are light bending as it leaves the water.
Dispersion is white light splitting into seven colours through a prism — VIBGYOR: violet, indigo, blue, green, yellow, orange, red. A rainbow is the same effect, with raindrops acting as tiny prisms.
The eye is a natural camera, with a convex lens that focuses an image on a light-sensitive screen.
👁️ Part and job
| Part | What it does |
|---|---|
| Cornea | the clear front window where light enters |
| Iris | the coloured ring that controls the pupil |
| Pupil | the opening that lets light in — it widens in the dark |
| Lens | a convex lens that focuses the light |
| Retina | the screen at the back where the image forms |
| Optic nerve | carries the message to the brain |
The image on the retina is upside down. The brain turns it the right way up. This surprises students and is asked directly.
Two common defects. Short sight (myopia) — distant things blur; corrected with a concave lens. Long sight (hypermetropia) — near things blur; corrected with a convex lens.
12 questions on reflection, refraction and the eye. 🍀
