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 Syllabus ref: 2.2 Electromagnetic energy travels at the speed of light in the form of waves. These waves are oscillations in the electrical field and the corresponding magnetic field which is at right angles to it. Electromagnetic waves travel at the speed of light. The wave may be characterised by its wavelength and frequency.

Wave definitions

Waves are defined by velocity, wavelength, frequency and amplitude. This is shown in the following diagram:

1. The wavelength is represented by the Greek letter lambda - l
2. The frequency is the number of wavelengths that pass per second - f (sometimes the Greek letter 'nu', n). This is measured in Hertz (Hz) or cycles per second.
3. The velocity of the wave in the case of electromagnetic radiation is the speed of light 'c' (3 x 108 ms-1 [data booklet])
4. The amplitude of the wave is represented by the letter 'a'. It is a measure of the intensity and is relatively unimportant.

These wave properties are related by the equation:

Electromagnetic energy

The energy carried by an electromagnetic wave depends on the wavelength - they are inversely proportional, i.e. the shorter the wavelength the higher the energy.

However, as the speed of the electromagnetic radiation is always constant, the energy is also directly proportional to the frequency of the wave. Higher frequency = greater energy. This can be shown by the following equation:

Where 'h' is called Planck's constant (6.63 x 10-34 Js [data booklet])

 Example: Calculate the frequency of radiation emitted by an IR source with a wavelength of 1500 nm. The speed of light, c = lf Hence: f = c/l = 3 x 108/1500 x 10-9 Hence frequency of radiation = 2 x 1014 Hz

The electromagnetic spectrum

The wavelength of electromagnetic radiation varies according to its energy. The table below shows the terms used for different regions of electromagnetic radiative energy and their approximate wavelength and frequency ranges.

 Frequency Wavelength Type of electromagnetic radiation Energy 1024 3 x 10-16 Gamma rays high to low 1021 3 x 10-13 far X-rays 1018 3 x 10-10 X-rays 1015 < 3 x 10-7 Ultraviolet 1015 3.6 - 7.6 x 10-7 Visible 1015 > 3 x 10-7 Infrared 1012 3 x 10-4 Microwaves 109 3 x 10-1 UHF television 106 3 x 102 Radio waves

Note that the visible region of the spectrum, i.e. the region that human eyes are able to detect, is incredibly small when compared to the whole range.

Effectively, there is no upper or lower limit to the wavelength and the energy.