7.4 Electromagnetic spectrum
- Syllabus
- 9702–2028–2029
- Topic
- 7.4
- Level
- AS
Learning objectives
7.4.1That all electromagnetic waves are transverse waves that travel with the• state that all electromagnetic waves are transverse waves that travel with the same speed c in free space7.4.2The approximate range of wavelengths in free space of the principal regions• recall the approximate range of wavelengths in free space of the principal regions of the electromagnetic spectrum from radio waves to γ-rays7.4.3That wavelengths in the range 400–700 nm in free space are visible to the• recall that wavelengths in the range 400–700 nm in free space are visible to the human eye
All electromagnetic waves are transverse and travel at c in free space
Electromagnetic waves consist of mutually perpendicular electric and magnetic fields, both transverse to the direction of travel, and move at c≈3.00×10⁸ m s⁻¹ in vacuum.
They do not require a material medium; in matter their speed can be lower and depends on the medium.
Radio, visible light and X-rays are different frequencies of the same transverse electromagnetic family.
“Electromagnetic” does not mean longitudinal, and c applies to free space, not every material.
Anchor each electromagnetic region to an approximate wavelength range
| Region (long λ → short λ) | Approximate free-space wavelength |
|---|---|
| radio | longer than 10⁻¹ m |
| microwave | 10⁻³ m to 10⁻¹ m |
| infrared | 7 × 10⁻⁷ m to 10⁻³ m |
| visible | 4 × 10⁻⁷ m to 7 × 10⁻⁷ m |
| ultraviolet | 10⁻⁸ m to 4 × 10⁻⁷ m |
| X-ray | 10⁻¹¹ m to 10⁻⁸ m |
| gamma | shorter than 10⁻¹¹ m |
From radio to gamma, wavelength decreases while frequency increases because c = fλ in free space. The spectrum is continuous, so quoted boundaries are approximate conventions and may overlap slightly between sources.
Convert first, then classify. Useful anchors are 1 mm = 10⁻³ m, 1 μm = 10⁻⁶ m and 1 nm = 10⁻⁹ m. Thus 2.1 cm = 2.1 × 10⁻² m is microwave, 12 μm = 1.2 × 10⁻⁵ m is infrared, and 138 pm = 1.38 × 10⁻¹⁰ m is X-ray.
If frequency is given, calculate λ = c/f before using the table. For f = 3.0 × 10¹⁶ Hz, λ = 1.0 × 10⁻⁸ m, at the approximate ultraviolet/X-ray boundary; use the convention and options supplied by the question.
Order alone does not satisfy a wavelength-range question. Do not reverse the trend: gamma has the shortest wavelengths and radio the longest. Approximate region labels do not imply gaps in the spectrum.
Visible light occupies roughly 400–700 nm in free space
The visible band is approximately 400–700 nm in free space, from shorter-wavelength violet to longer-wavelength red.
Treat the limits as approximate and use wavelength/frequency trends when identifying colour or neighbouring ultraviolet and infrared.
A 500 nm wave lies in the visible range, while 350 nm is ultraviolet and 900 nm is infrared.
Visible colour is not determined by intensity alone; wavelength sets the nominal colour while intensity affects brightness.