What is the Electromagnetic Spectrum?
Electromagnetic (EM) waves are waves that transfer energy without needing a medium (they can travel through a vacuum). They all travel at the speed of light:
The EM spectrum is the full range of EM radiation ordered by wavelength (or frequency).
The Seven Types
From longest wavelength / lowest frequency → shortest wavelength / highest frequency:
| Type | Wavelength (approx.) | Uses | Dangers |
|---|---|---|---|
| Radio waves | > 1 m | Radio/TV broadcasting | None at normal levels |
| Microwaves | 1 mm – 1 m | Microwave ovens, Wi-Fi, satellites | Can heat body tissue |
| Infrared (IR) | 700 nm – 1 mm | TV remotes, thermal cameras, heaters | Burns (high intensity) |
| Visible light | 400 – 700 nm | Seeing, photography | None (normal levels) |
| Ultraviolet (UV) | 10 – 400 nm | Sun tanning, sterilisation | Skin cancer, eye damage |
| X-rays | 0.01 – 10 nm | Medical imaging, security scanners | Cell damage, cancer |
| Gamma rays | < 0.01 nm | Cancer treatment, sterilising food | Very dangerous — cell damage |
A useful mnemonic: Rather Many Ignored Very Useful X-tra items Globally (Radio, Micro, Infrared, Visible, UV, X-ray, Gamma)
Key Relationships
As wavelength increases, frequency decreases (and vice versa):
where = speed of light, = frequency (Hz), = wavelength (m).
Higher frequency = higher energy. Gamma rays carry the most energy; radio waves carry the least.
Visible Light
Visible light is the tiny portion of the EM spectrum that human eyes detect, with wavelengths 400–700 nm: Violet → Blue → Green → Yellow → Orange → Red
Real world: Microwave oven in the kitchen
A microwave oven emits microwaves at about 2.45 GHz. Water molecules in food absorb this radiation and vibrate faster — heating the food from within. The metal walls of the oven reflect microwaves back (preventing leakage), and the mesh on the door has gaps smaller than the microwave wavelength (~12 cm), so microwaves cannot escape but visible light can pass through.
Real world: X-rays at the hospital
When you break a bone, a hospital uses X-rays (wavelength 0.01–10 nm) to image it. X-rays pass easily through soft tissue but are partially absorbed by dense bone — so the bone appears white on the X-ray image. This non-invasive technique has been routine in medicine since the late 1800s.
Real world: TV remote control (infrared)
Your TV remote uses infrared (IR) radiation to send signals to the TV. When you press a button, the remote's LED flashes an IR pulse code. The TV's IR receiver detects it and responds. You can't see the flashes, but your phone camera can — point the remote at a camera and press a button to see the IR light flicker.
Practice Problems
Which part of the EM spectrum has the shortest wavelength?
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Why can EM waves travel through space but sound cannot?
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A microwave has a frequency of 2.4 GHz. Calculate the wavelength. (c = 3 × 10⁸ m/s)