Light Waves and the Electromagnetic Spectrum
Light consists of electromagnetic (EM) waves. An EM wave is composed of an electric field and a magnetic field that are oscillating together. The fields are oriented perpendicular to each other, and the wave travels in a direction perpendicular to both of the fields (see image at right). These waves can also be thought of as particles called photons: massless packets of energy that travel at the speed of light. In fact, EM radiation behaves as both a particle and a wave at the same time. EM waves can be characterized by any of three properties: wavelength (λ) – the distance between two adjacent crests of the wave, frequency (f) – the number of wave oscillations per second, or the energy (E) of the individual photons in the wave. For all types of EM radiation, the simple relationships between wavelength, frequency, and energy are:
where wavelength is measured in units of length such as meters (where 1 cm = 10-2 meters, 1 micrometer = 10-6 meters, etc.), frequency is measured in units of Hertz (Hz), where 1 Hz = 1 wave crest per second (e.g. 1 MHz = 106 Hz, 1 GHz = 109 Hz); c is the speed of light, which is about 3 x 108 meters per second (or 186,000 miles per second); and h is Planck’s constant, which is equal to 6.63 x 10-27 erg/s, where an erg is a unit of energy. Remarkably, all forms of EM radiation (visible light, x-rays, radio waves, etc.) travel at the speed of light, regardless of their energy. Since the energy of an EM wave is directly proportional to its frequency and inversely proportional to its wavelength, the higher the energy of the wave, the higher the frequency, and the shorter the wavelength.
The different wavelengths of EM radiation cause the radiation to react differently with different materials, such as our eyes or detectors in telescopes. The way visible light of different wavelengths interacts with our eyes gives rise to “colors”, with the shorter wavelengths (about 0.0004 mm) appearing as blue light and the longer wavelengths (about 0.0007 mm) appearing as red light. Even shorter wavelengths of EM radiation (such as x-rays) can pass right through tissues in our bodies. Radiation at longer wavelengths (e.g. infrared) cannot be seen by our eyes, but can be felt as heat. Radio waves are EM waves with the longest wavelengths, from 1 mm – 100 km. The image below shows the entire electromagnetic spectrum, from shorter wavelengths to longer wavelengths.