Electromagnetic Waves — Part 2: The electromagnetic spectrum: visual lesson, ISC Class 12 Physics

Lesson

The electromagnetic spectrum

Part 2 — seven bands, one kind of wave: from radio waves kilometres long to gamma rays smaller than an atom.

In this lesson you will

Arrange the regions of the electromagnetic spectrum in order of frequency or wavelength, with approximate ranges
Give a source, a detector and a use for each region
Use c = fλ to place a wave in its band, and explain everyday applications from radar to radiotherapy

Concept

In order of increasing frequency (decreasing wavelength): radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.

The boundaries are approximate; neighbouring bands overlap.
X-rays and gamma rays of the same wavelength are identical; they are named by their origin (electrons for X-rays, nuclei for gamma rays).

Table of approximate wavelength and frequency ranges of the seven bands, from radio waves longer than 30 centimetres to gamma rays shorter than about 10 picometres; the boundaries are not sharp
Band	Wavelength (about)	Frequency (about)
Radio	longer than 30 cm	below 1 GHz
Microwave	1 mm to 30 cm	1 GHz to 300 GHz
Infrared	700 nm to 1 mm	3 × 10¹¹ to 4.3 × 10¹⁴ Hz
Visible	400 nm to 700 nm	4.3 × 10¹⁴ to 7.5 × 10¹⁴ Hz
Ultraviolet	1 nm to 400 nm	7.5 × 10¹⁴ to 3 × 10¹⁷ Hz
X-rays	1 pm to 1 nm	3 × 10¹⁷ to 3 × 10²⁰ Hz
Gamma rays	below about 10 pm (overlaps X-rays)	above about 3 × 10¹⁹ Hz

Diagram

Every band obeys $c = f\lambda$ in vacuum, so $f$ rises as $\lambda$ falls. On a logarithmic scale each step is a factor of 1000 in wavelength.

Visible light is a very narrow band, about 400–700 nm.
From ultraviolet upwards the radiation can ionise atoms and damage living cells.