P6: Light and the Electromagnetic Spectrum

What you'll learn

This topic covers the properties and behaviour of electromagnetic waves, including reflection, refraction and the complete electromagnetic spectrum. You'll learn how different EM waves are used in communication, medicine and everyday applications, plus the dangers associated with certain wavelengths and how to calculate wave speed.

Key terms and definitions

Electromagnetic wave — a transverse wave consisting of oscillating electric and magnetic fields that can travel through a vacuum at 3 × 10⁸ m/s

Wavelength — the distance between two consecutive points in phase on a wave, measured in metres (m)

Frequency — the number of complete waves passing a point per second, measured in hertz (Hz)

Reflection — when a wave bounces off a surface; the angle of incidence equals the angle of reflection

Refraction — the change in direction of a wave as it passes from one medium to another due to a change in speed

Ionising radiation — electromagnetic radiation with enough energy to remove electrons from atoms, potentially damaging living cells

Specular reflection — reflection from a smooth surface where parallel rays remain parallel after reflection

Diffuse reflection — reflection from a rough surface where parallel rays scatter in different directions

Core concepts

The electromagnetic spectrum

The electromagnetic spectrum consists of seven types of electromagnetic wave, all travelling at the same speed in a vacuum (3 × 10⁸ m/s) but with different wavelengths and frequencies.

Order from longest wavelength to shortest:

• Radio waves (wavelengths >10 cm)
• Microwaves (1 mm – 10 cm)
• Infrared (700 nm – 1 mm)
• Visible light (400 nm – 700 nm)
• Ultraviolet (10 nm – 400 nm)
• X-rays (0.01 nm – 10 nm)
• Gamma rays (<0.01 nm)

Key relationships:

• As wavelength decreases across the spectrum, frequency increases
• As frequency increases, the energy of the wave increases
• Higher energy waves (X-rays and gamma rays) are ionising and therefore more dangerous

Visible light spectrum:

Within visible light, different wavelengths correspond to different colours. From longest to shortest wavelength: red, orange, yellow, green, blue, indigo, violet (often remembered as "Richard Of York Gave Battle In Vain").

Wave speed calculations

All electromagnetic waves obey the wave equation:

wave speed = frequency × wavelength

v = f × λ

Where:

• v = wave speed in metres per second (m/s)
• f = frequency in hertz (Hz)
• λ = wavelength in metres (m)

For electromagnetic waves in a vacuum or air, v = 3 × 10⁸ m/s.

Important unit conversions:

• 1 nm (nanometre) = 1 × 10⁻⁹ m
• 1 μm (micrometre) = 1 × 10⁻⁶ m
• 1 mm (millimetre) = 1 × 10⁻³ m
• 1 kHz (kilohertz) = 1 × 10³ Hz
• 1 MHz (megahertz) = 1 × 10⁶ Hz
• 1 GHz (gigahertz) = 1 × 10⁹ Hz

Reflection of light

Reflection occurs when light bounces off a surface. Two laws govern reflection:

The law of reflection:

• The angle of incidence = the angle of reflection
• Both angles are measured from the normal (an imaginary line perpendicular to the surface at the point of incidence)

Types of reflection:

Specular reflection occurs from smooth surfaces like mirrors or still water. Parallel incident rays produce parallel reflected rays, forming clear images.

Diffuse reflection occurs from rough surfaces like paper or walls. Parallel incident rays scatter in many directions because the normal varies across the uneven surface. Each ray still obeys the law of reflection locally.

Drawing ray diagrams:

1. Draw the reflecting surface as a straight line
2. Draw the normal perpendicular to the surface at the point of incidence
3. Draw the incident ray approaching the surface
4. Measure the angle of incidence from the normal
5. Draw the reflected ray at an equal angle on the opposite side of the normal
6. Label all angles and rays clearly

Refraction of light

Refraction is the change in direction of light when it passes from one medium to another with a different density.

Key principles:

• Light slows down when entering a denser medium (e.g., air to glass)
• Light speeds up when entering a less dense medium (e.g., glass to air)
• When light slows down, it bends towards the normal
• When light speeds up, it bends away from the normal
• Light travelling along the normal does not change direction

Refractive index:

The refractive index (n) indicates how much a material slows down light:

n = speed of light in vacuum / speed of light in material

Higher refractive index = denser material = greater bending of light

Common values: air ≈ 1.0, water ≈ 1.33, glass ≈ 1.5

Applications:

• Lenses in glasses, cameras and telescopes use refraction to focus light
• Optical fibres use total internal reflection (occurs when light attempts to exit a dense medium at a shallow angle)
• Prisms separate white light into colours because different wavelengths refract by slightly different amounts

Uses and dangers of electromagnetic waves

Radio waves:

• Uses: television and radio broadcasting, communications
• Dangers: none at normal exposure levels

Microwaves:

• Uses: satellite communications, mobile phones, microwave ovens (heat water molecules in food)
• Dangers: internal heating of body tissue at high intensities

Infrared:

• Uses: thermal imaging, remote controls, optical fibre communications, cooking (grills and toasters)
• Dangers: skin burns at high intensities

Visible light:

• Uses: vision, photography, illumination, optical fibres
• Dangers: very bright light can damage the retina

Ultraviolet:

• Uses: fluorescent lamps, security marking, disinfection (kills bacteria)
• Dangers: skin cancer, sunburn, eye damage (cataracts); UV is ionising at higher frequencies

X-rays:

• Uses: medical imaging, airport security scanners, detecting flaws in materials
• Dangers: ionising radiation causes cell damage and mutation; risk of cancer

Gamma rays:

• Uses: sterilising medical equipment, cancer radiotherapy, detecting cancer
• Dangers: highly ionising; causes severe cell damage, radiation sickness and cancer

Protection measures:

• Lead shielding for X-rays and gamma rays (very dense material absorbs radiation)
• Short exposure times for medical imaging
• Sunscreen and protective clothing for UV exposure
• Standing behind barriers when X-ray equipment operates
• Lead aprons for radiographers

Investigating infrared radiation

The absorption and emission of infrared depends on surface properties:

Dark, matt surfaces:

• Good absorbers of infrared radiation
• Good emitters of infrared radiation

Light, shiny surfaces:

• Poor absorbers of infrared radiation (good reflectors)
• Poor emitters of infrared radiation

Practical investigation:

Students may be asked about experiments comparing different surfaces. A typical setup involves:

1. Heating identical objects with different surface finishes (e.g., matt black and shiny metal cans)
2. Placing the objects equal distances from a heat source
3. Monitoring temperature rise using thermometers or temperature sensors
4. Controlling variables: same volume of water, same initial temperature, same time

Expected results:

• Matt black surfaces heat up faster (better absorbers)
• Matt black surfaces cool down faster (better emitters)

Changes and wave behaviour

Colour and absorption:

Objects appear coloured because they reflect certain wavelengths and absorb others:

• A red object reflects red light and absorbs all other colours
• A white object reflects all wavelengths
• A black object absorbs all wavelengths

Filters:

Coloured filters transmit certain wavelengths and absorb others:

• A green filter transmits green light and absorbs other colours
• White light through a red filter appears red
• Red light through a blue filter appears black (no transmission occurs)

Worked examples

Example 1: Wave speed calculation

Question: A radio station broadcasts at a frequency of 95.8 MHz. Calculate the wavelength of the radio waves. (Speed of light = 3 × 10⁸ m/s) [3 marks]

Solution:

Convert frequency to Hz: 95.8 MHz = 95.8 × 10⁶ Hz = 9.58 × 10⁷ Hz [1 mark]

Rearrange v = f × λ to give: λ = v / f [1 mark]

λ = (3 × 10⁸) / (9.58 × 10⁷) = 3.13 m [1 mark]

Mark scheme notes: Award 1 mark for correct unit conversion, 1 mark for correct equation or rearrangement, 1 mark for correct final answer with unit.

Example 2: Refraction

Question: A ray of light travels from air into a glass block. The angle of incidence is 40° and the angle of refraction is 25°. Explain what happens to the speed and direction of the light. [4 marks]

Solution:

The light slows down [1 mark] as it enters the denser medium/glass [1 mark].

The light bends towards the normal [1 mark] because it has slowed down / entered a denser medium [1 mark].

Mark scheme notes: Accept "speed decreases" for slowing down. Must link bending towards normal to change in speed or density for full marks.

Example 3: EM spectrum application

Question: Explain why gamma rays are used to sterilise medical equipment but are dangerous to humans. [4 marks]

Solution:

Gamma rays have high energy/frequency / short wavelength [1 mark] and can penetrate materials/kill bacteria and microorganisms [1 mark].

Gamma rays are ionising radiation [1 mark] which can damage cells/DNA and cause mutations/cancer [1 mark].

Mark scheme notes: Must explain both why they are useful (penetration/killing microbes) and why they are dangerous (ionising/cell damage). Credit biological consequences.

Common mistakes and how to avoid them

• Confusing refraction and reflection — Reflection is bouncing off a surface; refraction is bending when entering a different medium. Always check which process the question asks about.

• Measuring angles from the surface instead of the normal — Always measure angles of incidence and reflection from the perpendicular normal line, not from the surface itself.

• Mixing up the EM spectrum order — Create a mnemonic for the order from long to short wavelength. Remember that radio waves have the longest wavelength, gamma rays the shortest.

• Forgetting unit conversions in calculations — Always convert frequencies to Hz and wavelengths to metres before using v = f × λ. Check prefixes carefully (MHz, GHz, nm, μm).

• Stating X-rays and gamma rays are "harmful" without explaining ionising — To gain full marks, explain that these waves are ionising radiation that removes electrons from atoms, causing cell damage and mutations.

• Drawing refraction diagrams with light bending the wrong way — When light enters a denser medium it slows down and bends towards the normal; when it exits to a less dense medium it speeds up and bends away from the normal.

Exam technique for "P6: Light and the Electromagnetic Spectrum"

• "Describe" questions require you to state what happens without explanation. For refraction: "The light bends towards the normal" is sufficient. "Explain" questions require reasons: "The light bends towards the normal because it slows down when entering the denser medium."

• Ray diagrams must be drawn with a ruler and include labels for: incident ray, reflected/refracted ray, normal, and relevant angles. Arrows show direction of light travel. Typically worth 3-4 marks with marks awarded for correct normal, accurate angles, and appropriate labels.

• Calculation questions always require: the correct equation (often given), substitution of values with units, and a final answer with the correct unit. Show your working clearly — if you make an arithmetic error, you can still gain method marks.

• 6-mark extended response questions might ask you to compare uses and dangers of different EM waves. Structure your answer by dealing with each wave type systematically, covering both use and danger for full marks. Use scientific vocabulary precisely.

Quick revision summary

The electromagnetic spectrum consists of seven wave types ordered by wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All travel at 3 × 10⁸ m/s in a vacuum. Reflection occurs when waves bounce off surfaces following the law of reflection. Refraction is wave bending due to speed changes between media. Higher energy waves (UV, X-rays, gamma) are ionising and dangerous. Each wave type has specific uses: radio for broadcasting, microwaves for communications, infrared for thermal applications, visible for sight, UV for sterilisation, X-rays for imaging, gamma for cancer treatment.