3.3 Electromagnetic Spectrum

Electromagnetic waves are a fundamental part of the electromagnetic spectrum. This spectrum encompasses a wide range of waves, all characterized by their wavelength and frequency. A key property of all electromagnetic waves is that they travel at the same speed in a vacuum.

Speed of Light in a Vacuum

The speed at which electromagnetic waves travel in a vacuum is a fundamental physical constant, denoted by $c$. Its approximate value is $2.998 \times 10^8 \, m/s$. This speed is constant regardless of the motion of the source or the observer.

Properties of Electromagnetic Waves

• Wavelength (λ): The distance between two successive crests or troughs of a wave.
• Frequency (f): The number of complete wave cycles passing a point per unit time. Measured in Hertz (Hz).
• Amplitude: The maximum displacement of a wave from its equilibrium position.

The relationship between the speed ($c$), frequency ($f$), and wavelength ($\lambda$) of an electromagnetic wave is given by the following equation:

$$c = f \times \lambda$$

The Electromagnetic Spectrum

The electromagnetic spectrum is conventionally divided into different regions based on the wavelengths and frequencies of electromagnetic waves. These regions include:

• Radio Waves: Longest wavelengths, lowest frequencies. Used in communication (radio, television).
• Microwaves: Shorter wavelengths than radio waves, higher frequencies. Used in cooking, communication.
• Infrared (IR) Radiation: Shorter wavelengths than microwaves, higher frequencies. Associated with heat.
• Visible Light: The portion of the spectrum that the human eye can detect. Different wavelengths correspond to different colours.
• Ultraviolet (UV) Radiation: Shorter wavelengths than visible light, higher frequencies. Can cause sunburn.
• X-rays: Shorter wavelengths than UV radiation, higher frequencies. Used in medical imaging.
• Gamma Rays: Shortest wavelengths, highest frequencies. Produced by radioactive decay.

Region	Wavelength Range (approximate)	Frequency Range (approximate)	Common Uses
Radio Waves	1 mm - 100 km	3 kHz - 300 GHz	Communication, broadcasting
Microwaves	1 mm - 1 m	300 MHz - 300 GHz	Cooking, communication
Infrared (IR)	700 nm - 1 mm	300 GHz - 430 THz	Heat, remote controls
Visible Light	400 nm - 700 nm	430 THz - 750 THz	Vision, photography
Ultraviolet (UV)	10 nm - 400 nm	750 THz - 30 PHz	Sterilisation, tanning
X-rays	0.01 nm - 10 nm	30 PHz - 30 EHz	Medical imaging
Gamma Rays	< 0.01 nm	> 30 EHz	Radioactive decay

It is crucial to understand that all these different regions of the electromagnetic spectrum are simply different forms of the same fundamental wave – electromagnetic radiation – and they all propagate at the same speed in a vacuum.