Nolan

"Fundamentals of college Physics, Second Edition, Peter J. Nolan, State University of New York at Farmingdale"

25.10 The Electromagnetic Spectrum

We have seen that electromagnetic waves exist and propagate through space at the speed of light. We represent the electric wave by equation 15.38, and the electric field vector E depends on the wavelength and the frequency of the wave. The wavelength and frequency are not independent but are related by the fundamental equation of wave propagation, equation 12.3 with the speed ᶹ replaced by c, and frequency f replaced by the Greek lower case letter ᶹ (nu). Thus,

                                 c = ƛ ᶹ                                                (25.58)

The use of letter ᶹ for the frequency of the electromagnetic wave rather than the letter f, that we used previously when waves were described in chapter 12, is customary in physics when dealing with electromagnetic radiation in modern physics.

It is evident that an entire series of electromagnetic waves should exist, differing only in frequency and wavelength. Such a group of electromagnetic waves has been found and are divided into six main categories: radio waves, infrared waves, visible light waves, ultraviolet waves, X rays, and gamma rays. The entire group of electromagnetic waves is called electromagnetic spectrum. Let us look at some of the characteristics of these waves.

1. Radio Waves. Radio waves are usually described in terms of their frequency AM (amplitude modulated) radio waves are emitted at frequency from 550 kHz to 1600 kHz. ( Recall that the unit kHz is a kilohertz, which is a thousand cycles per second, hence 550 kHz is equal to 550 X 10³ cycles per second or 5.50 X 10⁵ cycles /s.) FM (frequency modulated) radio waves, on the other hand, are transmitted in the range off 88 MHz to 108 MHz (Recall that MHz is a megahertz which is equal to 10⁶ Hz.) Television waves are transmitted in the range of 44 MHz to 216 MHz. Ultra-High frequency (UHF) TV waves are broadcast in the range of 470 MHz to 890 MHz. Microwaves, which are used in radar sets and microwave ovens fall in the range of 1 GHz to 30 GHz. A gigahertz (GHz) is equal to 10⁹  cycles/s.

Satellite television antennas are used to receive both waves (VHF) and ultra-high frequency television waves (UHF).

2. Infrared Waves. Infrared waves are usually described in terms of their wavelength rather than their frequency. The infrared spectrum extends from approximately 720 nm to 50,000 nm. Recall that the unit nm is nanometer and equal to 10^-9 m. Infrared frequency can be determined from equation 25.58.

3. Visible light. Visible light occupies a very small portion of the electromagnetic spectrum, from 380 nm to 720 nm. The wavelength of 380 nm corresponds to a violet color, while 720 nm corresponds to red color.

4. Ultraviolet Light. The ultraviolet portion of spectrum extends from around 10 nm up to about 380 nm. It is this ultraviolet radiation from the sun that causes sunburn and skin cancer.

5. X Rays. X rays are very energetic electromagnetic waves. They are usually formed when high-speed charged particles are brought to rest on impact with matter. The x-ray portion of electromagnetic spectrum lies in the range 0.01 nm up to about 150 nm.

6. Gamma Rays. Gamma rays are the most energetic of all the electromagnetic waves and fall in the range of almost 0 to 0.1 nm overlapping the x-ray region. They differ from X rays principally in origin. They are emitted from the nucleus of an atom, whereas X rays are usually associated with processes occurring in the electron shell structure of the atom.