# Electromagnetic Spectrum

The electromagnetic spectrum* describes the range of energies associated with different forms of electromagnetic radiation. Electromagnetic radiation travels through space as discrete packets called photons. Photons can transport energy the way particles do, but photons have no mass*. Photons vary in the amount of energy they carry. The energy associated with a photon determines where on the electromagnetic spectrum it falls.

In the illustration below, drag the light blue slider to change the wavelength* shown.

As photons travel through space, they behave like waves. Like waves, they have

• a speed (C) in the direction they are traveling
• a wavelength (λ), which is the distance between equivalent points on neighboring waves.
• a frequency* (f), which is the number of times the photon oscillates per second and
• discrete amounts of energy (E).
Metric units are used to describe each of these characteristics. Wavelengths are measured in meters. Frequency is measured in cycles or oscillations per second. Speed, the distance traveled over time, is measured in meters per second

Electromagnetic energy is measured in electron volts. As a unit of energy, electron volt values can be converted to joules, the standard SI unit of energy.

The speed of electromagnetic radiation does not vary. In a vacuum, photons travel at the constant speed of 3x108 m/s. Wavelength, frequency and energy values vary over huge ranges. Wavelengths range from 10’s to 100’s of meters for radio waves down to less than 1 pm, or 10^-12 meters for gamma rays.

Radiation with the shortest wavelengths have the highest frequencies. These frequencies can be greater than 1020 cycles per second for gamma rays. Longer wavelength radiation such as radio waves, have lower frequencies. These are in the range of millions of cycles per second.

There is a straightforward mathematical relationship between frequency, wavelength and speed: wavelength times frequency equals speed and speed is constant at 3x108

C = λ × f

The mathematical relationship between energy, speed and wavelength is energy equals Planck’s constant (h) times speed divided by the wavelength:

E = h x C / λ

The radiation we perceive as visible light* covers a narrow range in the middle of the spectrum. The longest wavelengths of light are red. The shortest wavelengths light are violet. Above red on the spectrum are microwaves followed by Radio waves. Below violet light is ultraviolet, followed by x-rays then gamma rays.

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