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Electromagnetic (EM) waves are transverse waves consisting of oscillating electric and magnetic fields, perpendicular to each other and to the direction of propagation. They require no medium and all travel at the speed of light in a vacuum: c = 3.00 × 10⁸ m s⁻¹.
All electromagnetic waves share the following properties:
The electromagnetic spectrum is a continuous range of wavelengths and frequencies. We divide it into named regions for convenience, but there are no sharp boundaries between them.
The following table summarises the approximate wavelength range, frequency range, and key properties of each region. Wavelength decreases and frequency increases from top to bottom.
| Region | Wavelength Range | Frequency Range | Typical Source |
|---|---|---|---|
| Radio waves | > 0.1 m | < 3 × 10⁹ Hz | Oscillating charges in aerials/antennae |
| Microwaves | 1 mm – 0.1 m | 3 × 10⁹ – 3 × 10¹¹ Hz | Magnetrons, klystrons |
| Infrared (IR) | 700 nm – 1 mm | 3 × 10¹¹ – 4.3 × 10¹⁴ Hz | Hot objects, heaters |
| Visible light | 400 nm – 700 nm | 4.3 × 10¹⁴ – 7.5 × 10¹⁴ Hz | Very hot objects, LEDs, lasers |
| Ultraviolet (UV) | 10 nm – 400 nm | 7.5 × 10¹⁴ – 3 × 10¹⁶ Hz | The Sun, UV lamps, very hot stars |
| X-rays | 0.01 nm – 10 nm | 3 × 10¹⁶ – 3 × 10¹⁹ Hz | Decelerating electrons hitting a metal target |
| Gamma rays (γ) | < 0.01 nm | > 3 × 10¹⁹ Hz | Radioactive decay of unstable nuclei |
Exam Tip: You must know the order of the EM spectrum and typical wavelength/frequency ranges. A common mnemonic is: Running Mice In Valleys Use X-tra Grass.
Within the visible spectrum, wavelengths range from approximately:
| Colour | Approximate Wavelength |
|---|---|
| Red | 620 – 700 nm |
| Orange | 590 – 620 nm |
| Yellow | 570 – 590 nm |
| Green | 495 – 570 nm |
| Blue | 450 – 495 nm |
| Violet | 380 – 450 nm |
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