The Electromagnetic Spectrum

Key Words

-Amplitude Modulation -Electromagnetic Spectrum -Frequency Modulation -Thermograms

Objectives

-Rank and classify electromagnetic waves based on their frequencies and wavelengths -Describe the uses for different waves of the electromagnetic spectrum

How do electromagnetic waves vary?

Electromagnetic waves vary by the length of their waves and exist on an electromagnetic spectrum.

Rank the following list of waves from shortest to longest wavelength: radio, visible light, gamma, UV, infrared.

From shortest to longest: gamma, UV, visible light, infrared, radio

Gamma Rays

Gamma rays have the shortest wavelengths in the electromagnetic spectrum, about 0.005 nanometer or less. They have the highest frequencies and therefore the most energy and the greatest penetrating ability of all the electromagnetic waves. Exposure to tiny amount of gamma rays are tolerable, but overexposure can be deadly. Gamma rays are used in the medical field to kill cancer cells and make pictures of the brain, and in industrial situations as an inspection tool. Gamma rays are used in radiation therapy to kill cancer cells without harming nearby healthy cells. Gamma rays are also used to make pictures of the human brain, with different levels of brain activity represented by different colors. Pipelines are checked with machines that travel on the inside of a pipe, taking gamma ray pictures along the entire length. Technicians examine the pictures for rusting, cracks, or other signs of damage.

The Waves of the Spectrum

In England in 1800, with a technique discovered earlier, Herschel used a prism to separate the wavelengths present in sunlight. He produced a band of colors: red, orange, yellow, green, blue, and violet. He wondered if the temperature of each color of light was different from the temperature of the other colors of light. Herschel placed thermometers at various places along the color band and measured the temperatures. Herschel observed that the temperature was lower at the blue end and higher toward the red end. This discovery made Herschel pose a new question: Would the temperature increase even more beyond the red end, in an area that showed no color? He measured the temperature just beyond the red end of the color band. This area recorded an even higher temperature than the red area. Herschel concluded there must be an invisible radiation beyond the red end of the color band.

Radio

In a radio studio, music and voices that have been changed into electronic signals are coded onto radio waves and then broadcast. There are two ways that radio stations code and transmit information on radio waves. Both ways are based on a wave of constant frequency and amplitude. To code the information onto this wave so that it can be broadcast, one of two characteristics of the wave must be varied, or modulated.

Amplitude Modulation/Frequency Modulation

In amplitude modulation, the amplitude of the wave is varied. The frequency remains the same. AM radio stations broadcast by amplitude modulation. In frequency modulation, the frequency of the wave is varied. The amplitude remains the same. FM stations broadcast by frequency modulation. Whichever way the radio wave is transmitted, your radio receives it, decodes it, and changes it back into sound waves you can hear. Have you ever traveled a long distance in a car and "lost" a station on the radio? A station is lost when its signal becomes too weak to detect. An FM radio station is more likely to be lost than an AM station because FM radio signals do not travel as far as AM signals along Earth's curved surface. AM radio stations use frequencies between 535 kilohertz and 1605 kilohertz. FM stations use frequencies between 88 megahertz and 108 megahertz. Particles in Earth's upper atmosphere reflect the lower-frequency AM radio waves much better than the higher-frequency FM radio waves. The reflection helps transmit AM signals farther.

Describe how people can use infrared radiation.

Infrared radiation is just outside the visible light spectrum and can be detected only by their source of heat. Infrared radiation is used to detect heat differences in objects or to keep things war. Thermograms are pictures that are generated by these heat differences.

Infrared Rays

Infrared rays have a higher frequencies than radio waves and lower frequencies than red light. Infrared wavelengths vary from about 1 millimeter to about 750 nanometers. (A nanometer is 10^-9 meters, or one millionth of a millimeter.) Infrared rays are used as a source of heat and to discover areas of heat differences. You cannot see infrared radiation, but your skin senses it as warmth. Reptile habitats at zoos are often kept warm with infrared lamps. Restaurants use infrared lamps to keep buffet-style foods at a safe temperature for consumption.

Why do scientists have to use satellites and other pieces of technology to learn about most of the electromagnetic waves?

Only a small percentage of the electromagnetic spectrum is visible, called the visible light spectrum. Waves that are shorter and waves that are longer must be detected by technology, such as satellites.

Television

Radio waves also carry signals for television rogramming. The process is like transmitting radio signals. But one difference is that the radio waves carry information for pictures as well as for sound. Once they have been broadcast, the signals are received by an antenna, and sent to the TV set. Location and weather can affect the reception of television signals by an antenna. For that reason, many people prefer to receive television signals that have been transmitted by satellite. With this type of transmission, TV broadcasts are sent to satellites, which then retransmit the signals back to Earth. If you have a satellite dish, you can receive the signals directly. If not, a cable service can receive the signals and resend them to your home.

Radio Waves

Radio waves have the longest wavelengths in the electromagnetic spectrum, from 1 millimeter to as much as thousands of kilometers or longer. Because they are the longest waves, radio waves also have the lowest frequencies in the spectrum--300,000 megahertz (MHz) or less. Radio waves are used in radio and television technologies, as well as in microwave ovens and radar.

Electromagnetic Spectrum

The full range of frequencies of electromagnetic radiatoin is called the electromagnetic spectrum. Visible light is the only part of the electromagnetic spectrum that you can see, but it is just a small part. The electromagnetic spectrum includes radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. Each kind of wave is characterized by a range of wavelengths and frequencies. All of these waves have many useful applications.

Microwaves

The shortest-wavelength radio waves are called microwaves. Microwaves have wavelengths from about 1 meter to about 1 millimeter. Their frequencies vary from about 300 megahertz to about 300,000 megahertz. Microwaves cook and reheat food. When water or fat molecules in the food absorb microwaves, the thermal energy of these molecules increases. But microwaves generally penetrate foods only a few centimeters, so heating occurs only near the surface of the food. That is why instructions tell you to let the food stand for a few minutes--so thermal energy can reach the center by conduction. Microwaves also carry cell phone conversations. The process works much like the radio broadcast. This process works much like the radio broadcast.

Visible Light

The visible part of the electromagnetic spectrum is light that the human eye can see. Each wavelength in the visible spectrum corresponds to a specific frequency and has a particular color. People use visible light to see, to help keep them safe, and to communicate with one another. Light enables people to read. It is what makes flowers, boxes, signs, and all other objects visible. Automobiles have headlights and taillights that make night driving safer. Traffic lights communicate information to drivers about what is expected of them--to stop, for example, when the light is red.

Ultraviolet Rays

The wavelengths of ultraviolet rays vary from about 400 nanometers to about 4 nanometers. Ultraviolet radiation has higher frequencies than violet light. Ultraviolet rays have applications in health and medicine, and in agriculture. In moderation, exposure to ultraviolet rays helps your skin produce vitamin D. Vitamin D helps the body absorb calcium from foods to produce healthy bones and teeth. Excessive exposure can cause sunburn, wrinkles, and eventually skin cancer. It can also damage your eyes. Ultraviolet rays are used to kill microorganisms. In heating and cooling systems of large buildings, ultraviolet rays disinfect the air that flows through the systems. In winter, plant nurseries use ultraviolet lights to help plants grow.

Radar

The word radar is an acronym for radio detection and ranging. Radar technology uses a radio transmitter to send out short bursts or radio waves. The waves reflect off the objects they encounter, and bounce back toward where they came from. The returning waves are then picked up and interpreted by a radio receiver. Recall that the Doppler effect is an apparent change in the frequency of a wave. The Doppler effect can be used to find the speed of a moving car. Radio waves are sent from a stationary source, such as a radar trailer, toward a moving car. The faster a car is moving toward the source, the higher is the frequency of the radio waves returning to the source.

Infrared Radiation

Today, radiation beyond the red end of the color band is called infrared radiation. Herschel experimented with infrared radiation and found it had many of the same properties as visible light. With these experiments, Herschel opened the door to the study of invisible types of electromagnetic radiation.

Thermograms

Warmer objects give off more infrared radiation than cooler objects. A device called a thermograph uses infrared sensors to create thermograms. Thermograms are color-coded pictures that show variations in temperature. They are used to find places where a building loses heat to the environment. Thermograms can also locate problems in the path of electric current. The human body is usually warmer than its surroundings. After a natural disaster such as an earthquake, search-and-rescue teams use infrared camercas to locate victims quickly--even underground.

X-Rays

X-rays have very short wavelengths, from about 12 nanometers to about 0.005 nanometers. They have higher frequencies than ultraviolet rays. X-rays have high energy and can penetrate matter that light cannot. X-rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects. Your teeth and bones absorb X-rays. X-ray photographs show softer tissues as dark, highly exposed areas. Bones and teeth appear white. Too much exposure to X-rays can kill or damage living tissue. The lids on aluminum cans are sometimes inspected with X-rays to make sure they are sealed properly. X-rays can be used to identify the contents of entire truck trailers. Packages and suitcases are X-rayed in search of dangerous contents.