The big idea: The warmth of sunlight on your skin, the signal reaching your phone, the X-ray of a broken bone — all are the same thing on the move: an electromagnetic (EM) wave, a ripple of vibrating electric and magnetic fields.
Every EM wave is transverse, and in a vacuum they all travel at the same speed, c = 3.00 × 10⁸ m s⁻¹.
The EM spectrum is the whole family, sorted by wavelength (and so by frequency).
New words: Wavelength λ — the length of one full wave (m). Frequency f — how many waves pass each second (hertz, Hz).
Transverse — the wave's vibration is across (perpendicular to) the way it travels. Vacuum — empty space, no air or material.
| Region | Wavelength | Frequency | Everyday use |
|---|---|---|---|
| Radio | longest | lowest | TV, radio, phone signals |
| Microwave | ↓ | ↑ | ovens, wifi, radar |
| Infrared (IR) | ↓ | ↑ | heat, remote controls, night vision |
| Visible | ≈ 400–700 nm | ↕ | the light your eyes see |
| Ultraviolet (UV) | ↓ | ↑ | suntan, sterilising |
| X-ray | ↓ | ↑ | seeing bones |
| Gamma (γ) | shortest | highest | from nuclei, cancer treatment |
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Remember the order: Radio → Micro → Infrared → Visible → Ultraviolet → X-ray → Gamma.
Going that way: wavelength gets shorter, frequency gets higher, and energy gets higher. A handy phrase: Rock Music Is Very Useful for eXtra Groove.
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For any wave, the speed equals the frequency times the wavelength. This is the wave equation, and it is given in the data booklet.
- wave speed (m s⁻¹) — for EM waves in vacuum this is c
- frequency (Hz) — waves passing each second
- wavelength (m) — length of one full wave
For EM waves the speed is c: In a vacuum every EM wave travels at the speed of light, c = 3.00 × 10⁸ m s⁻¹ (a given constant).
So for EM waves the wave equation becomes:
c = f λ
Rearrange it to find whichever one is missing.
- speed of light in vacuum = 3.00 × 10⁸ m s⁻¹ (a given constant)
- frequency (Hz)
- wavelength (m)
c = speed of light, f = frequency, λ = wavelength. Cover the one you want: two side by side → multiply; one above the other → divide.
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A microwave used by a phone mast has a wavelength of 0.15 m in air. Treating the speed as c = 3.00 × 10⁸ m s⁻¹, find its frequency.
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How this is tested — EM-spectrum questions are quick identify / outline marks:
Paper 1A
- Given a wavelength or frequency, name the region (e.g. λ ≈ one atom across → an X-ray).
- State that EM waves are transverse.
Paper 2
- Outline a difference between sound and EM waves — e.g. EM waves travel through a vacuum but sound needs a medium.
The classic trap: Thinking different colours or regions travel at different speeds. In a vacuum they all travel at c.
Sound waves (mechanical)
- Longitudinal (vibrate along the travel direction)
- Need a medium — cannot cross a vacuum
- Speed ≈ 340 m s⁻¹ in air (much slower)
- It is the air particles that oscillate
EM waves (e.g. light)
- Transverse (vibrate across the travel direction)
- Travel through a vacuum — no medium needed
- Speed = c = 3.00 × 10⁸ m s⁻¹ in vacuum
- It is electric & magnetic fields that oscillate
Why f against 1/λ is a straight line: Since c = f λ, dividing by λ gives f = c × (1/λ) — the form y = (slope) x.
So a graph of f against 1/λ is a straight line through the origin whose slope is c, the same for every EM region.
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A physicist studies an EM wave whose wavelength is about 1 × 10⁻¹⁰ m (roughly the diameter of a single atom). Using c = 3.00 × 10⁸ m s⁻¹, find the wave's frequency and state which region of the EM spectrum it belongs to.
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