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This lesson covers the electromagnetic (EM) spectrum — the full range of electromagnetic waves — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to know the order of the spectrum, the properties shared by all EM waves, and the approximate wavelength and frequency ranges.
Electromagnetic (EM) waves are waves that consist of oscillating electric and magnetic fields. They are fundamentally different from mechanical waves (such as sound) because they do not require a medium to travel through.
Exam Tip: A key exam point: all EM waves travel at the same speed in a vacuum (3 × 10⁸ m/s). The difference between the types is their wavelength and frequency. Higher frequency = shorter wavelength (because v = fλ and v is constant).
EM waves are generated by changes in atoms and their nuclei:
The EM spectrum is a continuous range of wavelengths and frequencies. The seven main regions, listed in order of increasing frequency (and decreasing wavelength), are:
| Region | Wavelength Range | Frequency Range | Example Source |
|---|---|---|---|
| Radio waves | > 1 m (up to km) | < 3 × 10⁸ Hz | Radio transmitters, TV stations |
| Microwaves | 1 mm – 1 m | 3 × 10⁸ – 3 × 10¹¹ Hz | Microwave ovens, satellites |
| Infrared (IR) | 700 nm – 1 mm | 3 × 10¹¹ – 4.3 × 10¹⁴ Hz | Hot objects, heaters, remote controls |
| Visible light | 400 nm – 700 nm | 4.3 × 10¹⁴ – 7.5 × 10¹⁴ Hz | The Sun, light bulbs, LEDs |
| Ultraviolet (UV) | 10 nm – 400 nm | 7.5 × 10¹⁴ – 3 × 10¹⁶ Hz | The Sun, UV lamps |
| X-rays | 0.01 nm – 10 nm | 3 × 10¹⁶ – 3 × 10¹⁹ Hz | X-ray tubes in hospitals |
| Gamma rays (γ) | < 0.01 nm | > 3 × 10¹⁹ Hz | Radioactive nuclei |
A common mnemonic to remember the order (from longest wavelength to shortest):
Runny Monkeys In Vests Use X-tra Grease
(Radio, Microwave, Infrared, Visible, Ultraviolet, X-rays, Gamma)
Visible light is the small portion of the EM spectrum that the human eye can detect. It ranges from red (longest wavelength, lowest frequency) to violet (shortest wavelength, highest frequency):
| Colour | Approximate Wavelength |
|---|---|
| Red | ~700 nm |
| Orange | ~620 nm |
| Yellow | ~580 nm |
| Green | ~530 nm |
| Blue | ~470 nm |
| Violet | ~400 nm |
Mnemonic: Richard Of York Gave Battle In Vain (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
Since all EM waves travel at the same speed in a vacuum (c = 3 × 10⁸ m/s), there is an inverse relationship between wavelength and frequency:
c = f × λ
Calculate the frequency of a radio wave with a wavelength of 1500 m.
f = c / λ
f = (3 × 10⁸) / 1500
f = 2 × 10⁵ Hz = 200 kHz
Calculate the wavelength of an X-ray with a frequency of 3 × 10¹⁸ Hz.
λ = c / f
λ = (3 × 10⁸) / (3 × 10¹⁸)
λ = 1 × 10⁻¹⁰ m = 0.1 nm
Exam Tip: When calculating with EM waves, always use c = 3 × 10⁸ m/s for the speed. Make sure wavelength is in metres (not nm or mm) before substituting into the equation. Remember: 1 nm = 1 × 10⁻⁹ m.
The energy carried by an EM wave is directly related to its frequency:
This means:
This relationship between frequency and energy is why higher-frequency EM radiation is more dangerous to living tissue (discussed in the next lesson).
| Property | EM Waves | Sound Waves |
|---|---|---|
| Type | Transverse | Longitudinal |
| Medium required? | No (can travel through vacuum) | Yes (requires a medium) |
| Speed in vacuum | 3 × 10⁸ m/s | Cannot travel |
| Speed in air | ~3 × 10⁸ m/s | ~330 m/s |
| Examples | Light, radio, X-rays, gamma | Speech, music, ultrasound |
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