Refraction & Lenses
Year 9 🌊 Waves & Optics Explain refraction using Snell's law; describe converging and diverging lenses.
🔭 Snell's Law and Refractive Index
When light crosses a boundary between two materials, it bends. The refractive index (n) measures how much a material bends light.
🔭 Snell's Law
$$n_1 \sin\theta_1 = n_2 \sin\theta_2 \qquad n = \frac{c}{v} = \frac{\sin\theta_i}{\sin\theta_r}$$| 🧪 Material | 📊 Refractive index (n) |
|---|---|
| Vacuum / Air | 1.00 |
| Water | 1.33 |
| Crown glass | 1.52 |
| Diamond | 2.42 |
🔦 Example: Light hits glass (n=1.5) at 30° to the normal. Find refraction angle:
sin θ₂ = (1.0 × sin30°) / 1.5 = 0.5/1.5 = 0.333 → θ₂ = 19.5°
sin θ₂ = (1.0 × sin30°) / 1.5 = 0.5/1.5 = 0.333 → θ₂ = 19.5°
🔍 Converging (Convex) Lenses
A converging lens is thicker in the middle. It refracts parallel rays to meet at the focal point (F).
| 📍 Object position | 🖼️ Image type | 📏 Image size | 🔍 Use |
|---|---|---|---|
| Beyond 2F | Real, inverted | Smaller | Camera, eye |
| At 2F | Real, inverted | Same size | Photocopier |
| Between F and 2F | Real, inverted | Larger | Projector |
| At F | No image (rays parallel) | — | Lighthouse, torch |
| Inside F | Virtual, upright | Larger | Magnifying glass! |
📷 Diverging (Concave) Lenses
A diverging lens is thinner in the middle. It spreads parallel rays outward, as if they came from a virtual focal point.
Diverging lenses always produce virtual, upright, and smaller images — no matter where the object is placed.
🔭 Thin Lens Formula
$$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \qquad m = \frac{v}{u}$$
$f$ = focal length (m) · $v$ = image distance (m) · $u$ = object distance (m, negative)
👓 Spectacles: Short-sighted people use diverging lenses to spread light before it enters the eye — making distant objects appear clear!
Ready to test yourself? Click the Quiz tab above to answer questions on this topic!
🔭 Snell's Law & Lens Calculator
Lens Formula: 1/f = 1/v + 1/u