The Electromagnetic Spectrum

This lesson covers the electromagnetic (EM) spectrum — the continuous range of electromagnetic waves from radio waves to gamma rays — as required by the AQA GCSE Physics specification (4.6.2). You need to know the order of the EM spectrum, the properties shared by all EM waves, and how wavelength and frequency change across the spectrum.

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What Are Electromagnetic Waves?

Electromagnetic (EM) waves are transverse waves that transfer energy from one place to another. Unlike sound waves, EM waves do not need a medium — they can travel through a vacuum (empty space).

Key properties of all EM waves:

• They are all transverse waves.
• They all travel at the same speed in a vacuum: 3 x 10^8 m/s (the speed of light).
• They can all travel through a vacuum.
• They all transfer energy.
• They can all be reflected, refracted, and absorbed.

Exam Tip: All electromagnetic waves travel at the same speed in a vacuum (3 x 10^8 m/s). This is a key fact that examiners love to test. The speed only changes when EM waves enter a medium (e.g. glass or water), where they slow down.

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The Electromagnetic Spectrum — Order

The EM spectrum is arranged in order of wavelength (and therefore frequency). From longest wavelength (lowest frequency) to shortest wavelength (highest frequency):

graph LR
    R["Radio waves"] --> Mi["Microwaves"] --> IR["Infrared"] --> V["Visible light"] --> UV["Ultraviolet"] --> X["X-rays"] --> G["Gamma rays"]

    style R fill:#e74c3c,color:#fff
    style Mi fill:#e67e22,color:#fff
    style IR fill:#f39c12,color:#fff
    style V fill:#27ae60,color:#fff
    style UV fill:#2980b9,color:#fff
    style X fill:#8e44ad,color:#fff
    style G fill:#2c3e50,color:#fff

EM Wave	Wavelength Range	Frequency Range
Radio waves	Greater than 1 m (up to km)	Less than 3 x 10^8 Hz
Microwaves	1 mm to 1 m	3 x 10^8 to 3 x 10^11 Hz
Infrared	700 nm to 1 mm	3 x 10^11 to 4.3 x 10^14 Hz
Visible light	400 nm to 700 nm	4.3 x 10^14 to 7.5 x 10^14 Hz
Ultraviolet	10 nm to 400 nm	7.5 x 10^14 to 3 x 10^16 Hz
X-rays	0.01 nm to 10 nm	3 x 10^16 to 3 x 10^19 Hz
Gamma rays	Less than 0.01 nm	Greater than 3 x 10^19 Hz

Memory Aid

A common mnemonic for remembering the order (from longest to shortest wavelength) is:

Running Men In Vests Usually X-ray Giraffes

(Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma rays)

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Wavelength, Frequency and Energy

Across the EM spectrum:

Property	From radio waves to gamma rays
Wavelength	Decreases (radio = longest, gamma = shortest)
Frequency	Increases (radio = lowest, gamma = highest)
Energy	Increases (radio = least energy, gamma = most energy)

The relationship between wavelength and frequency:

v = f x lambda

Since all EM waves travel at the same speed in a vacuum (v = 3 x 10^8 m/s), if the frequency increases, the wavelength must decrease (and vice versa). They are inversely proportional.

Exam Tip: A common question asks: "What happens to the frequency as the wavelength decreases?" The answer is: frequency increases. They are inversely proportional because v = f x lambda and v is constant for EM waves in a vacuum. Always state this relationship using the wave equation.

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Visible Light

Visible light is the only part of the EM spectrum that can be detected by the human eye. White light is a mixture of all the colours of the visible spectrum.

The colours of visible light, from longest wavelength to shortest:

Colour	Approximate Wavelength (nm)
Red	700
Orange	620
Yellow	580
Green	530
Blue	470
Indigo	420
Violet	400

Remember: Richard Of York Gave Battle In Vain (ROY G BIV)

• Red light has the longest wavelength and lowest frequency in the visible spectrum.
• Violet light has the shortest wavelength and highest frequency in the visible spectrum.

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Production of EM Waves

EM waves are produced by changes in atoms and nuclei:

EM Wave	How It Is Produced
Radio waves	Oscillating electrons in an aerial/antenna
Microwaves	Oscillating electrons in a magnetron
Infrared	Warm and hot objects emit infrared radiation
Visible light	Very hot objects (e.g. filament in a light bulb, the Sun)
Ultraviolet	The Sun; UV lamps; very hot objects
X-rays	High-speed electrons hitting a metal target
Gamma rays	Radioactive decay of unstable atomic nuclei

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Detection of EM Waves

EM Wave	How It Is Detected
Radio waves	Aerial / antenna connected to a receiver
Microwaves	Microwave detector / receiver
Infrared	Infrared detector, thermometer, skin (felt as warmth)
Visible light	Human eye, photographic film, CCD (camera sensor)
Ultraviolet	Fluorescent materials glow; UV-sensitive detector
X-rays	Photographic film, fluorescent screen, digital detector
Gamma rays	Geiger-Muller tube, photographic film

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Ionising and Non-Ionising Radiation

EM waves can be classified as ionising or non-ionising:

• Non-ionising: radio waves, microwaves, infrared, visible light — these do not have enough energy to remove electrons from atoms.
• Ionising: ultraviolet (partially), X-rays, gamma rays — these do have enough energy to remove electrons from atoms, which can damage cells and DNA.

graph LR
    subgraph "Non-Ionising"
        R2["Radio"] --- M2["Micro"] --- I2["IR"] --- V2["Visible"]
    end
    subgraph "Ionising"
        U2["UV"] --- X2["X-rays"] --- G2["Gamma"]
    end
    style R2 fill:#27ae60,color:#fff
    style M2 fill:#27ae60,color:#fff
    style I2 fill:#27ae60,color:#fff
    style V2 fill:#27ae60,color:#fff
    style U2 fill:#e74c3c,color:#fff
    style X2 fill:#e74c3c,color:#fff
    style G2 fill:#e74c3c,color:#fff

Exam Tip: The boundary between ionising and non-ionising is not sharp. Ultraviolet is sometimes considered partially ionising (UV-B and UV-C are ionising; UV-A is not). For AQA GCSE, you should know that UV, X-rays, and gamma rays are ionising and can damage living cells.

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Worked Example

A radio station broadcasts at a frequency of 98.5 MHz. Calculate the wavelength of the radio waves.

Step 1: Convert frequency to Hz.

f = 98.5 MHz = 98.5 x 10^6 Hz = 9.85 x 10^7 Hz

Step 2: Use the wave equation.