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Electromagnetic Waves

by Ron Kurtus (revised 30 December 2001)

Although they seem different, radio waves, microwaves, x-rays, and even visible light are all waves of energy called electromagnetic waves. They are part of the electromagnetic spectrum, and each has a different range of wavelengths, which cause they waves to affect matter differently. The electromagnetic waves have amplitude, wavelength, velocity and frequency. The creation and detection of the wave depend much on the range of wavelengths.

Questions you may have include:

• What is the electromagnetic spectrum?
• What are the characteristics of electromagnetic waves?
• How are these waves created and detected?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Time = 6 min. 52 sec.

Electromagnetic spectrum

The range of wavelengths for electromagnetic waves--from the very long to the very short--is called the Electromagnetic Spectrum:

• Radio and TV waves are the longest usable waves, having a wavelength of 1 mile (1.5 kilometer) or more.
• Microwaves are used in telecommunication as well as for cooking food.
• Infrared waves are barely visible. They are the deep red rays you get from a heat lamp.
• Visible light waves are the radiation you can see with your eyes. Their wavelengths are in the range of 1/1000 centimeter.
• Ultraviolet rays are what give you sunburn and are used in "black lights" that make object glow.
• X-rays go through the body and are used for medical purposes.
• Gamma rays are dangerous rays coming from nuclear reactors and atomic bombs. They have the shortest wavelength in the electromagnetic spectrum of about 1/10,000,000 centimeter.

Characteristics of electromagnetic waves

Electromagnetic waves are transverse waves, similar to water waves in the ocean or the waves seen on a guitar string. This is as opposed to the compression waves of sound. As you learned in Wave Motion, all waves have amplitude, wavelength, velocity and frequency.

Amplitude

The amplitude of electromagnetic waves relates to its intensity or brightness (as in the case of visible light).

With visible light, the brightness is usually measured in lumens. With other wavelengths the intensity of the radiation, which is power per unit area or watts per square meter is used. The square of the amplitude of a wave is the intensity.

Wavelength

The wavelengths of electromagnetic waves go from extremely long to extremely short and everything in between. The wavelengths determine how matter responds to the electromagnetic wave, and those characteristics determine the name we give that particular group of wavelengths.

Velocity

The velocity of electromagnetic waves in a vacuum is approximately 186,000 miles per second or 300,000 kilometers per second, the same as the speed of light. When these waves pass through matter, they slow down slightly, according to their wavelength.

Frequency

The frequency of any waveform equals the velocity divided by the wavelength. The units of measurement are in cycles per second or Hertz.

Creation and detection

When electrons move, they create a magnetic field. When electrons move back and forth or oscillate, their electric and magnetic fields change together, forming an electromagnetic wave. This oscillation can come from atoms being heated and thus moving about rapidly or from alternating current (AC) electricity.

The opposite effect occurs when an electromagnetic wave hits matter. In such a case, it could cause atoms to vibrate, creating heat, or it can cause electrons to oscillate, depending on the wavelength of the radiation.

Sources of electromagnetic radiation

Electromagnetic radiation is emitted from all matter with a temperature above absolute zero. Temperature is the measure of the average energy of vibrating atoms and that vibration causes them to give off electromagnetic radiation. As the temperature increases, more radiation and shorter wavelengths of electromagnetic radiation are emitted.

Sources of long wavelengths

Microwaves, radio, and television waves are emitted from electronic devices. Sparks and alternating current cause vibrations at the appropriate frequencies.

Sources of visible light

Visible light is emitted from matter hotter than about 700 degrees Celsius. This matter is said to be incandescent. The sun, a fire, and the ordinary light bulb are incandescent sources of light.

As the element in an electric stove gets warms, it gives off infrared radiation, and then when it gets hotter than 700 degrees, it starts to glow. Visible light is being emitted from the hot element.

(See Visible Light for more information.)

Sources of short wavelengths

By smashing high-energy electrons into other particles, such as atoms in a metal, X-rays are created.

(See X-rays for more information.)

Gamma rays are emitted from nuclear reactions, atomic bombs, and explosions on the Sun and other stars. 

Detectors of electromagnetic radiation

There are a number of different types of detectors of electromagnetic radiation. We know the common ones for detecting visible light: the eye, camera film, and the detectors on some calculators. Your skin can also detect both visible light and infrared heat rays.

Electronic devices are necessary to detect most of the longer waves, such as radio waves. Special film can detect shorter wavelengths such as X-rays

Summary

Electromagnetic waves are transverse waves that travel at the speed of light. The spectrum of their wavelengths results in waves that are used in many of our useful devices. The waves are formed by heat, electronics, and nuclear forces.

Answers to Readers' Questions

Learn to ability to think logically

Resources

The following resources provide information on this subject:

Websites

Physical Science Resources

Books

Top-rated books on Electromagnetic Waves

Miscellaneous

Mini-quiz to check your understanding

1. What is the wavelength of an electromagnetic wave?

2. Which is NOT an electromagnetic wave?

3. How are radio waves and visible light similar?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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