Light and the Electromagnetic Spectrum
The Electromagnetic Spectrum
All EM waves travel at 3 × 10⁸ m/s in a vacuum (the speed of light, c).
From longest wavelength / lowest frequency to shortest wavelength / highest frequency:
Radio → Microwave → Infrared → Visible → UV → X-ray → Gamma
| Wave | Wavelength range | Key uses |
|---|---|---|
| Radio | > 0.1 m | TV/radio broadcasting, MRI |
| Microwave | 1 mm – 0.1 m | Cooking food, satellite communication, radar |
| Infrared (IR) | 700 nm – 1 mm | Thermal imaging, remote controls, optical fibres |
| Visible | 400–700 nm | Human vision, photography |
| Ultraviolet (UV) | 10–400 nm | Sterilisation, fluorescence, sun tanning/damage |
| X-ray | 0.01–10 nm | Medical imaging (bones), airport security |
| Gamma (γ) | < 0.01 nm | Killing cancer cells (radiotherapy), sterilisation of medical equipment |
Higher frequency → higher energy → more ionising → more dangerous to biological tissue.
Refraction and Refractive Index
When light crosses a boundary between media, its speed changes → wavelength changes → direction changes (unless normal incidence).
Snell's Law: n₁ sin θ₁ = n₂ sin θ₂
For air (n₁ ≈ 1) entering glass (n₂ = n): sin i / sin r = n
Refractive index: n = sin(angle of incidence) / sin(angle of refraction) = c / v (speed in medium)
Glass: n ≈ 1.5; water: n ≈ 1.33; air: n ≈ 1.0.
Total internal reflection (TIR): occurs when light travels from a denser to less dense medium at an angle greater than the critical angle C: sin C = 1/n
TIR is used in optical fibres (communications, medical endoscopes) and diamond cutting.
Lenses
Converging (convex) lens: brings parallel rays to a focus at the focal point (F). Used in cameras, magnifying glasses, eye corrections for long sight.
Diverging (concave) lens: spreads parallel rays so they appear to diverge from a virtual focal point. Used in correction of short sight.
Ray diagrams — three principal rays for a convex lens:
- Ray parallel to principal axis → refracts through focal point F.
- Ray through the optical centre → travels straight (undeviated).
- Ray through the near focal point → refracts parallel to principal axis.
Image properties: Real/virtual, upright/inverted, magnified/diminished — determined by object distance relative to F.
Core Practical 5 — Investigating refraction of light
Equipment: glass block (rectangular or semicircular), ray box, protractor, ruler, paper.
Method:
- Place the glass block on paper. Draw round it.
- Direct a light ray (single slit) at the flat face at various angles of incidence (20°–70°).
- Mark the incident and refracted rays. Measure angles from the normal.
- Calculate n = sin i / sin r for each angle. Average the values.
Key safety: ray boxes get hot — do not touch the glass envelope.
Edexcel examiner tip: A graph of sin i (y-axis) vs sin r (x-axis) should be a straight line through the origin; the gradient = n.
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