7.4 Electromagnetic spectrum

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.

The principal regions are radio, microwave, infrared, visible, ultraviolet, X-ray and gamma; wavelength decreases while frequency and photon energy increase across that order.

Use approximate ranges as scale guides, not as sharp universal borders. Match a region to an application only after checking the relevant wavelength or energy.

Microwaves have shorter wavelengths and higher frequencies than radio waves; X-rays are shorter and more energetic than ultraviolet.

The spectrum is continuous, and region labels are conventions rather than isolated gaps in wavelength.

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.