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The Electromagnetic Spectrum

Electromagnetism

Light is only a small part of a much wider family of waves. Radio, microwaves, infrared, ultraviolet, X-rays and gamma rays are all the same kind of wave, differing only in wavelength and frequency.

One family of waves

The electromagnetic spectrum is the full range of electromagnetic waves, ordered by wavelength or frequency. All of them are transverse waves made of oscillating electric and magnetic fields, and all travel at the speed of light in a vacuum. From longest wavelength to shortest, the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. The boundaries between regions are not sharp: they are labels for a continuous range. What changes along the spectrum is the energy carried by each photon and how the waves interact with matter, which is why different regions have such different uses.

Wavelength, frequency and energy

Wavelength and frequency are linked by the wave speed: frequency equals the speed of light divided by wavelength. A radio wave metres long has a frequency of millions of hertz, while a gamma ray with a wavelength smaller than an atom has a frequency above 10^19 Hz. Photon energy is proportional to frequency, so a single gamma-ray photon carries vastly more energy than a radio photon. This explains why ultraviolet, X-rays and gamma rays can damage cells and break chemical bonds, while radio waves and visible light at ordinary intensities cannot. The speed is the same for all of them in a vacuum; it only changes when they enter a material.

What each region is used for

Radio waves carry broadcasts, mobile signals and Wi-Fi, while microwaves heat food and link satellites. Infrared is used in remote controls, thermal cameras and heaters. Visible light is what our eyes detect, and ultraviolet helps make vitamin D but can also cause sunburn. X-rays pass through soft tissue and are used to image bones and inspect luggage, and gamma rays are used in cancer treatment and to sterilise medical equipment. Astronomers observe the whole spectrum, because different objects shine brightest in different regions: cool dust glows in infrared, while hot gas around black holes emits X-rays.

Common mistakes

• Thinking visible light is most of the spectrum: it is a tiny slice, roughly 400 to 700 nm.
• Confusing wavelength and frequency: as wavelength increases, frequency decreases, because their product is fixed at c.
• Assuming higher frequency always means more total energy: photon energy depends on frequency, but total energy also depends on how many photons there are.

Practice questions

1. Red light has wavelength 600 nm. What is its frequency?
   Show answer

   f = c/lambda = (3.0 x 10^8)/(6.0 x 10^-7) = 5.0 x 10^14 Hz.

2. An FM radio station broadcasts at 100 MHz. What is its wavelength?
   Show answer

   lambda = c/f = (3.0 x 10^8)/(1.0 x 10^8) = 3.0 m.

3. Which has more energy per photon, gamma rays or radio waves?
   Show answer

   Gamma rays, because they have the shortest wavelength and highest frequency, so each photon carries the most energy.

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