Electromagnetic waves and the spectrum

A source emits electromagnetic waves, which carry energy away from it. When an observer or another object absorbs the waves, energy is transferred to that absorber. An ‘observer’ need not be a person: it can be an object receiving the radiation.

For example, a hot heater emits infrared radiation. Your skin absorbs some of this radiation and warms up. The energy has travelled from the heater to your skin by electromagnetic waves.

In a microwave oven, microwaves emitted inside the oven are absorbed by water molecules in food. This transfers energy to the food, increasing its temperature.

Visible light also carries energy. Light emitted by the Sun can reach and be detected by your eyes, allowing you to see. Electromagnetic energy transfer does not always produce an obvious warming effect.

A continuous spectrum, divided into named groups

The electromagnetic spectrum is the complete, continuous range of electromagnetic waves. Its main groups, in order, are:

Radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays

These names divide up a continuous range; they are not seven isolated wavelengths with gaps between them.

Wavelength is the distance between corresponding points on consecutive waves, such as adjacent crests. Frequency is the number of complete oscillations each second. Because all electromagnetic waves have the same speed in a vacuum, a shorter wavelength corresponds to a higher frequency.

Moving from radio waves towards gamma rays, wavelength decreases and frequency increases. Radio waves have the longest wavelengths and lowest frequencies of these groups; gamma rays have the shortest wavelengths and highest frequencies.

The small part our eyes detect

Our eyes detect only a limited range of electromagnetic frequencies: visible light. Infrared and ultraviolet lie immediately outside this range, so neither is visible to the human eye.

Within visible light, the colours run in the order red, orange, yellow, green, blue, indigo, violet. Each colour corresponds to a narrow range of wavelengths and frequencies, with the colours blending continuously into one another.

Red is next to infrared and has the longest wavelength and lowest frequency of visible light. Violet is next to ultraviolet and has the shortest wavelength and highest frequency of visible light. The same wavelength–frequency pattern therefore applies both across the whole spectrum and within its visible part.

In this direction: wavelength decreases; frequency increases. The spectrum is continuous, with no gaps between the named groups.

Visible light

The human eye detects only a limited range of electromagnetic frequencies.

Red → orange → yellow → green → blue → indigo → violet

• Red: longest visible wavelength, lowest visible frequency.
• Violet: shortest visible wavelength, highest visible frequency.
• Infrared lies beyond red; ultraviolet lies beyond violet.