Chapter 13

Electromagnetic Waves

Waves are produced by something that vibrates, and they carry energy from one place to another. Whereas sound waves need a medium in order to travel, electromagnetic waves are able to travel without the presence of a medium. Instead of transferring energy from particle to particle, electromagnetic waves travel by transferring energy between vibrating electric and magnetic fields. Do you remember electric and magnetic fields from our previous chapters? When an electric charge vibrates, the electric field around it vibrates. Because the electric charge is in motion, it also has a magnetic field around it. This magnetic field is changing as the charge moves back and forth. As a result, the vibrating electric charge is surrounded by vibrating electric and magnetic fields. Electromagnetic waves travel outward from a vibrating charge in all directions. As an electromagnetic waves moves, its electric and magnetic fields encounter objects. These vibrating fields can exert forces on charged particles and magnetic materials, causing them to move. Look at the diagram below and notice the two vibrating fields:

Wave Speed - electromagnetic waves travel through space, which is empty, as well as through various materials-and they travel fast. In the time it takes you to blink your eyes, an electromagnetic wave can travel around the entire Earth (186,000 mph). Like all waves, electromagnetic waves can be described by their frequency and their wavelength. An electromagnetic wave can vibrate at different speeds, or frequencies. As wavelength decreases, frequency increases-what do you think happens to the amount of energy delivered?

Does energy travel as a wave or particle? - in 1887, Heinrich Hertz found that by shining light on a metal, electrons were ejected from the metal; this became known as the Photoelectric Effect. Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was pretty much discredited with Young's double slit experiment, which made things look as though light had to be a wave. But in the early 20th century, some physicists--Einstein, for one--began to examine the particle view of light again. Einstein noted that careful experiments involving the photoelectric effect could show whether light consists of particles or waves. If light behaves as a wave, the energy contained in one of those waves should depend only on its amplitude--that is, on the intensity of the light. Other factors, like the frequency, should make no difference. So, for example, red light and ultraviolet light of the same intensity should knock out the same number of electrons, and the maximum kinetic energy of both sets of electrons should also be the same. Decrease the intensity, and you should get fewer electrons, flying out more slowly; if the light is too faint, you shouldn't get any electrons at all, no matter what frequency you're using. The problem is, Heinrich's experiement showed that the frequency of light determined how many electrons were deflected; not the wave's amplitude. Years later, our hero Einstein provided an explanation for this event; he noted that high frequency light delivered more energy than low frequency light. In the Theory of Relativity, Einstein stated that light travels as both a wave and a particle. The electromagnetic wave delivers a particle of energy known as a "photon."

The Electromagnetic Spectrum - electromagnetic waves can have a wide variety of frequencies. They might vibrate once each second or trillions of times each second. The entire range of electromagnetic wave frequencies is known as the electromagnetic spectrum. The range of wavelengths for electromagnetic waves—from the very long to the very short is called the Electromagnetic Spectrum. Various portions of the spectrum interact with matter differently, as a result, they are given different names:

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 wavelength 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.

Type of Wave Description

Radiowave range: 1000 meters to 1 cm. Radio waves are found at the longest wavelengths on the electromagnetic spectrum. These are the waves that are used to send signals to your AM/FM Radio or your television (unless you have cable).

Microwave range: one-tenth of a mm to 1 cm. Microwaves are used in radar and also in your microwave appliance at home that you use for heating food.

Infrared Radiation is how we describe heat. We can't see infrared waves, but we can feel them. Your body gives off heat, so it is an emitter of infrared radiation. The range of infrared wavelengths is about sub-millimeters to micrometers (the size of a bacteria). The Visible Spectrum is the light that we can see, and thus is the only part of the electromagnetic spectrum that is detectable by the human eye. Visible light is described as white light, and it contains all the colors of the rainbow, from red to violet. The range of visible wavelengths is 400 to 700 nanometers. Ultraviolet light is the radiation from the sun that causes a sunburn when you have been outside too long on a sunny day. But, watch out! You can't see ultra-violet light, so you can still get sunburned on a cloudy day. The range for ultraviolet light is 10^-8 to 10^-10 meters.

X-rays are very energetic, and are used in X-ray machines to take pictures of your bones. The range for X-rays is 10^-10 to 10^-12 meters.

Gamma rays are the most energetic light waves found on the electromagnetic spectrum. We can find Gamma rays released in nuclear reactions and particle collisions. The range for a gamma ray is in picometers (10^-12 meters).

Radio communication is produced when radiowaves cause the electrons in an antenna to vibrate. These vibrating electrons produce an electric signal that contains information about the music and words produced by the radio station. An amplifier boosts the signal and sends it to speakers, causing them to vibrate at the same frequency. The vibrating speakers create sound waves that travel to your ears. Each radio station (or television station) is assigned one particular frequency that they can produce electromagnetic waves at. Turning the knob on your radio allows you to select a particular frequency to listen to. Radio stations must modify the waves one of two waves. If the amplitude of the wave is modified you have an AM (or amplitude modulated) wave, or if the frequency is modified you have a FM (or frequency modulated) wave. Radio stations can produce signals at low frequencies or high frequencies. An AM radio station brodcasts amplitude modulated waves that vibrate between 540,000 kilohertz (kilo means thousands) to 1.6 megahertz (mega means millions) vibrations per second. FM waves vibrate between 88 million times per second up to 108 million vibrations per second. Between AM waves [ending at 1.6 megahertz] and FM waves [beginning at 88 megahertz] are other frequencies: garage door remotes and alarm systems - 40 megahertz, cell phones - between 40 and 50 megahertz, television stations - between 50 and 80 megahertz.

Television and radio transmissions are similar, but television signals must include a video signal along with an audio signal. The audio transmission is sent by FM waves (frequency modulated) and video transmissions are sent by AM waves (amplitude modulated). These signals are sent as electromagnetic waves that vibrate millions of times per second. The audio signal is converted to sound by television speakers, the video signal is converted by a cathode-ray tube. A CRT is a sealed vacuum tube that produces three electron beams that are focused to a screen speckled with 100,000 rectangular red, blue, and green pixel cells. The CRT fires streams of electrons at the cells causing them to glow. An image is created when the three electron beams of the CRT sweep back and forth across the screen.

When you speak into a telephone a microphone converts sound waves in an electrical signal. In cell phones, this current is used to create radio waves that are transmitted to and from a microwave tower. The area in which a tower services is called a cell, which means signals must be relayed from tower to tower or for us . . . cell to cell. A cordless phone works much like a cell phone, but with a cordless phone you must stay near the base unit (the base unit serves as the first tower).

Additional assignments- print and turn in your work; you'll need to install ShockWave:
1. Online Practice Test
2. Interactive Tutor

Type of Wave	Description
	Radiowave range: 1000 meters to 1 cm. Radio waves are found at the longest wavelengths on the electromagnetic spectrum. These are the waves that are used to send signals to your AM/FM Radio or your television (unless you have cable).
	Microwave range: one-tenth of a mm to 1 cm. Microwaves are used in radar and also in your microwave appliance at home that you use for heating food.
	Infrared Radiation is how we describe heat. We can't see infrared waves, but we can feel them. Your body gives off heat, so it is an emitter of infrared radiation. The range of infrared wavelengths is about sub-millimeters to micrometers (the size of a bacteria). The Visible Spectrum is the light that we can see, and thus is the only part of the electromagnetic spectrum that is detectable by the human eye. Visible light is described as white light, and it contains all the colors of the rainbow, from red to violet. The range of visible wavelengths is 400 to 700 nanometers. Ultraviolet light is the radiation from the sun that causes a sunburn when you have been outside too long on a sunny day. But, watch out! You can't see ultra-violet light, so you can still get sunburned on a cloudy day. The range for ultraviolet light is 10^-8 to 10^-10 meters.
	X-rays are very energetic, and are used in X-ray machines to take pictures of your bones. The range for X-rays is 10^-10 to 10^-12 meters.
	Gamma rays are the most energetic light waves found on the electromagnetic spectrum. We can find Gamma rays released in nuclear reactions and particle collisions. The range for a gamma ray is in picometers (10^-12 meters).