Electromagnetic Spectrum

The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. This spectrum includes not only visible light (only a small portion of this spectrum) but also ultraviolet and infrared, microwave, radio and gamma waves. The sun, earth, and other bodies radiate electromagnetic energy of varying wavelengths. All electromagnetic energy passes through space at the speed of light (300 million meters per second) in the form of sinusoidal waves. Radiation is defined by its wavelength and frequency.

If a wave is passing by a point the wavelength is the distance between peaks in the wave, i.e. the distance from crest to crest. The frequency is the time interval between passing peaks. Therefore as the wavelength increases, the frequency decreases (and vice-versa).

Thus;

Frequency = Speed of Light / Wavelength   (Hz or Cycles per Second are the basic unit of Frequency)
Wavelength = Speed of Light / Frequency   (Micron is the basic unit for measuring the wavelength)

The spectrum of waves is divided into sections based on wavelength. The shortest waves are gamma rays, which have wavelengths of 10e-6 microns or less. The longest waves are radio waves, which have wavelengths of many kilometers. Visible light is a particular band of electromagnetic radiation that can be seen and sensed by the human eye. This energy consists of the narrow portion of the spectrum, from 0.4 microns (blue) to 0.7 microns (red). The infrared range starts at the end of the red spectrum with wavelengths greater than 0.7 microns. 

Usage Examples

Radio waves are used to transmit radio and television signals. Radio waves have wavelengths that range from less than a centimeter to tens or even hundreds of meters. FM radio waves are shorter than AM radio waves. For example, an FM radio station at 100 on the radio dial (100 megahertz) would have a wavelength of about three meters. An AM station at 750 on the dial (750 kilohertz) uses a wavelength of about 400 meters. Radio waves can also be used to create images. Radio waves with wavelengths of a few centimeters can be transmitted from a satellite or airplane antenna. The reflected waves can be used to form an image of the ground in complete darkness or through clouds.

Microwave wavelengths range from approximately one millimeter (the thickness of a pencil lead) to thirty centimeters (about twelve inches). In a microwave oven, the radio waves generated are tuned to frequencies that can be absorbed by the food. The food absorbs the energy and gets warmer. The dish holding the food doesn't absorb a significant amount of energy and stays much cooler. Microwaves are emitted from the Earth, from objects such as cars and planes, and from the atmosphere. These microwaves can be detected to give information, such as the temperature of the object that emitted the microwaves.

Infrared is the region of the electromagnetic spectrum that extends from the visible region to about one millimeter (in wavelength). Infrared waves include thermal radiation. For example, burning charcoal may not give off light, but it does emit infrared radiation which is felt as heat. Infrared radiation can be measured using electronic detectors and has applications in medicine and in finding heat leaks from houses. Infrared images obtained by sensors in satellites and airplanes can yield important information on the health of crops and can help us see forest fires even when they are enveloped in an opaque curtain of smoke.

The rainbow of colors we know as visible light is the portion of the electromagnetic spectrum with wavelengths between 400 and 700 billionths of a meter (400 to 700 nanometers). It is the part of the electromagnetic spectrum that we see, and coincides with the wavelength of greatest intensity of sunlight. Visible waves have great utility for the remote sensing of vegetation and for the identification of different objects by their visible colors.

Ultraviolet radiation has a range of wavelengths from 400 billionths of a meter to about 10 billionths of a meter. Sunlight contains ultraviolet waves which can burn your skin. Most of these are blocked by ozone in the Earth's upper atmosphere. A small dose of ultraviolet radiation is beneficial to humans, but larger doses cause skin cancer and cataracts. Ultraviolet wavelengths are used extensively in astronomical observatories. Some remote sensing observations of the Earth are also concerned with the measurement of ozone.

X-rays are high energy waves which have great penetrating power and are used extensively in medical applications and in inspecting welds. X-ray images of our Sun can yield important clues to solar flares and other changes on our Sun that can affect space weather. The wavelength range is from about ten billionths of a meter to about 10 trillionths of a meter.

Gamma rays have wavelengths of less than about ten trillionths of a meter. They are more penetrating than X-rays. Gamma rays are generated by radioactive atoms and in nuclear explosions, and are used in many medical applications. Images of our universe taken in gamma rays have yielded important information on the life and death of stars, and other violent processes in the universe.