Electromagnetic Radiation and Spectrum

Electromagnetic (EM) radiation is energy travelling through space as oscillating electric and magnetic fields. These fields are perpendicular to each other and to the direction of travel. EM radiation does not require a material medium and can travel through a vacuum.

EM waves are transverse waves, meaning their electric and magnetic fields oscillate at right angles to the direction of travel. All EM waves travel at approximately 3 × 10^8 metres per second in a vacuum, regardless of their frequency.

The electromagnetic spectrum is the complete range of EM radiation, arranged by wavelength or frequency. From lowest to highest frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Frequency and photon energy increase across this order, while wavelength decreases.

Radio waves occupy the low-frequency, long-wavelength end of the electromagnetic spectrum. They are non-ionising and are used for communication and radiofrequency applications, including excitation of hydrogen nuclei in MRI.

Microwaves have higher frequencies and shorter wavelengths than typical radio waves. They are non-ionising and are used in communications, heating and radar. They can transfer energy to materials and produce heating effects.

Infrared radiation lies between microwaves and visible light. It is non-ionising and is commonly associated with thermal radiation. It is used in thermal imaging and temperature-related applications.

Visible light is the small part of the electromagnetic spectrum detectable by the human eye. Its colours range from red, with longer wavelengths and lower frequencies, to violet, with shorter wavelengths and higher frequencies.

Ultraviolet (UV) radiation has shorter wavelengths and higher photon energies than visible light. UV can cause biological damage, including DNA damage. Some higher-energy UV photons can ionise certain substances, although UV is commonly grouped with non-ionising radiation in introductory classifications.

X-rays are high-energy electromagnetic photons capable of ionising atoms and molecules. They are commonly produced by interactions involving high-speed electrons. In radiotherapy, X-ray photons can deposit energy in tissue through interactions that release secondary electrons.

Gamma rays are electromagnetic photons generally produced during nuclear transitions or radioactive decay. They are often highly penetrating and can ionise matter. X-rays and gamma rays overlap in energy; their names usually distinguish their origin rather than a strict energy boundary.

Wavelength (λ) is the distance between matching points on successive waves, such as two neighbouring crests. It is measured in metres. Shorter wavelength corresponds to higher frequency for EM radiation travelling at the same speed.

Frequency (f) is the number of complete wave cycles passing a point each second. It is measured in hertz (Hz), where 1 Hz means one cycle per second. Higher-frequency EM radiation has greater photon energy.

Amplitude describes the maximum size of a wave's oscillation. For EM waves, it relates to the strength of the electric and magnetic fields. Greater field amplitude corresponds to greater intensity, but it does not automatically mean greater energy per photon.

The relationship between wave speed, frequency and wavelength is c = f × λ in a vacuum. Here c = 3 × 10^8 m/s, f is frequency in Hz and λ is wavelength in metres. Example: a frequency of 6 × 10^14 Hz gives a wavelength of 5 × 10^-7 m.

Electromagnetic radiation displays both wave-like and particle-like behaviour. Wave behaviour includes interference and diffraction. Particle behaviour occurs when radiation transfers energy in discrete packets called photons. Both descriptions are important for understanding X-ray interactions.

A photon is a discrete packet of electromagnetic energy. Photons have no electric charge and no rest mass. Each photon carries energy determined by its frequency. A higher-frequency photon has greater energy than a lower-frequency photon.

Photon energy is calculated using E = hf, where E is energy in joules, h is Planck's constant (6.626 × 10^-34 J·s) and f is frequency in hertz. Combining this with c = fλ gives E = hc/λ. Therefore, shorter wavelengths correspond to higher photon energies.

Ionising radiation has enough energy per interaction to remove an electron from an atom or molecule, creating ions. X-rays and gamma rays are ionising. Radio waves, microwaves, infrared and visible light are non-ionising. Ionisation can cause chemical and biological changes in tissue.

Intensity is the rate at which energy passes through a unit area, usually measured in W/m². Increasing the number of photons passing through an area each second increases intensity. Intensity is different from the energy carried by each individual photon.

Radiotherapy uses high-energy ionising radiation to deliver absorbed dose to tissues. Megavoltage X-ray beams interact with matter and generate energetic secondary electrons that deposit energy. Understanding photon energy, frequency, wavelength and intensity helps explain beam penetration and radiation interactions.