Section 1: What are electromagnetic waves?

• Waves in Space

  • Waves are produced by something that vibrates, and they carry energy from one place to another.

  • Electromagnetic Waves: made by vibrating electric charges and can travel through space where matter is not present.

    • Instead of transferring energy from particle to particle, electromagnetic waves travel by transferring energy between vibrating electric and magnetic fields.

• Electric and Magnetic Fields

  • Fields enable magnets and charges to exert forces at a distance. These fields extend throughout space.

  • Magnetic fields exist around magnets even if the space around the magnet contains no matter.

  • An electric field enables charges to exert forces on each other even when they are far apart.

  • Electric charges also can be surrounded by magnetic fields.

    • An electric current flowing through a wire is surrounded by a magnetic field.

  • An electric current in a wire is the flow of electrons in a single direction.

  • A changing magnetic field creates a changing electric field.

  • A changing electric field creates a changing magnetic field.

• Making Electromagnetic Waves

  • Electromagnetic waves also are produced when something vibrates—an electric charge that moves back and forth.

  • When an electric charge vibrates, the electric field around it changes.

  • A vibrating electric charge creates an electromagnetic wave that travels outward in all directions from the charge.

• Properties of Electromagnetic Waves

  • All matter contains charged particles that are always in motion.

  • As an electromagnetic wave moves, its electric and magnetic fields encounter objects.

  • As an electromagnetic wave strikes your skin, electrons in your skin gain energy from the vibrating electric and magnetic fields.

  • Radiant Energy: The energy carried by an electromagnetic wave.

  • All electromagnetic waves travel at 300,000 km/s in the vacuum of space.

  • Because light is an electromagnetic wave, the speed of electromagnetic waves in space is usually called the “speed of light.”

  • In matter, the speed of electromagnetic waves depends on the material they travel through.

  • The wavelength of an electromagnetic wave is the distance between the crests of the vibrating electric field or magnetic field.

  • The frequency of an electromagnetic wave also equals the frequency of the vibrating charge that produces the wave.

• Waves and Particles

  • A wave is a disturbance that carries energy, and a particle is a piece of matter.

  • Photon: Waves that behave as a particle whose energy depends on the frequency of the waves

  • When electrons are sent through two narrow slits, they behave as a wave.

Section 2: The Electromagnetic Spectrum

• A Range of Frequencies

  • Electromagnetic waves can have a wide variety of frequencies.

  • Electromagnetic waves are described by different names depending on their frequency and wavelength.

  • The entire range of electromagnetic wave frequencies is known as the electromagnetic spectrum.

    • Various portions of the electromagnetic spectrum interact with matter differently.

  • The electromagnetic waves that humans can detect with their eyes, called visible light, are a small portion of the entire electromagnetic spectrum.

• Radio Waves: low-frequency electromagnetic waves with wavelengths longer than about 1 mm.

  • A radio wave does not produce compressions and rarefactions as it travels through air.

  • Microwaves: Radio waves with wavelengths of less than about 30 cm.

    • Microwaves with wavelengths of about 1 cm to 20 cm are widely used for communication, such as for cellular telephones and satellite signals.

    • Microwave ovens use electromagnetic waves to heat food.

  • Another use for radio waves is to find the position and movement of objects by a method called radar.

    • Radar stands for RAdio Detecting And Ranging.

  • The amount of energy a proton releases depends on the type of tissue it is part of.

• Infrared Waves: a type of electromagnetic wave with wavelengths between about 1 mm and about 750 billionths of a meter.

  • Hotter objects emit more infrared waves than cooler objects emit. The wavelengths emitted also become shorter as the temperature increases.

  • Infrared images and visible light images can provide different types of information.

• Visible Light: the range of electromagnetic waves that you can detect with your eyes.

  • Visible light has wavelengths around 750 billionths to 400 billionths of a meter.

  • Your eyes contain substances that react differently to various wavelengths of visible light, so you see different colors.

    • These colors range from short-wavelength blue to long-wavelength red.

• Ultraviolet Waves: electromagnetic waves with wavelengths from about 400 billionths to 10 billionths of a meter.

  • Ultraviolet waves are energetic enough to enter skin cells.

    • Overexposure to ultraviolet rays can cause skin damage and cancer.

  • A useful property of ultraviolet waves is their ability to kill bacteria on objects such as food or medical supplies.

    • When ultraviolet light enters a cell, it damages protein and DNA molecules.

  • Fluorescent materials absorb ultraviolet waves and reemit the energy as visible light.

  • About 20 to 50 km above Earth’s surface in the stratosphere is a region called the ozone layer.

    • Ozone is a molecule composed of three oxygen atoms.

    • The ozone layer is vital to life on Earth because it absorbs most of the Sun’s harmful ultraviolet waves.

    • The chlorine atoms in CFCs react with ozone high in the atmosphere. This reaction causes ozone molecules to break apart.

• X Rays and Gamma Rays

  • The electromagnetic waves with the shortest wavelengths and highest frequencies are X rays and gamma rays.

  • X Rays: have wavelengths between about ten billionths of a meter and ten trillionths of a meter.

    • Doctors and dentists use low doses of X rays to form images of internal organs, bones, and teeth

  • Gamma Rays: Electromagnetic waves with wavelengths shorter than about 10 trillionths of a meter.

    • These are the highest- energy electromagnetic waves and can penetrate through several centimeters of lead.

    • Gamma rays are produced by processes that occur in atomic nuclei.

  • A beam of X rays or gamma rays can damage the biological molecules in living cells, causing both healthy and diseased cells to die.

Section 3: Radio Communication

• Radio Transmission

  • Radio waves exert a force on the electrons in an antenna, causing the electrons to vibrate.

  • Each radio station is assigned to broadcast at one particular radio frequency.

  • Carrier Wave: The specific frequency of the electromagnetic wave that a radio station is assigned.

    • The radio station must do more than simply transmit a carrier wave.

    • The station has to send information about the sounds that you are to receive. This information is sent by modifying the carrier wave.

  • A carrier wave broadcast by a radio station can be altered in one of two ways to transmit a signal: amplitude modulation (AM) or frequency modulation (FM).

  • An AM radio station broadcasts information by varying the amplitude of the carrier wave.

  • Electronic signals are transmitted by FM radio stations by varying the frequency of the carrier wave

• Television

  • Television and radio transmissions are similar.

  • Cathode-Ray-Tube: a sealed vacuum tube in which one or more beams of electrons are produced.

  • An image is created when the three electron beams of the CRT sweep back and forth across the screen.

  • By varying the brightness of each spot in a group, the three spots together can form any color so that you see a full-color image.

  • Cathode-raytubes produce the images you see on television. The inside surface of a television screen is covered by groups of spots that glow red, green, or blue when struck by an electron beam.

• Telephones

  • When you speak into a telephone, a microphone converts sound waves into an electrical signal.

  • A cell phone uses one radio signal for sending information to a tower at a base station. It uses another signal for receiving information from the base station.

  • Transceiver: transmits one radio signal and receives another radio signal from a base unit.

  • Another method of transmitting signals is a pager, which allows messages to be sent to a small radio receiver.

    • Each pager is given a unique number for identification.

• Communication Satellites

  • Communications satellites use solar panels to provide the electrical energy they need to communicate with receivers on Earth. The solar panels are the structures on either side of the central body of the satellite.

  • To avoid interference, the frequency broadcast by the satellite is different than the frequency broadcast from Earth.

  • To call on a mobile telephone, the telephone transmits radio waves directly to a satellite.

  • Satellite links work well for one-way transmissions, but two-way communications can have an annoying delay caused by the large distance the signals travel to and from the satellite.

  • The satellite-reception dishes that you sometimes see in yards or attached to houses are receivers for television satellite signals.

  • Communications satellites use microwaves rather than the longer-wavelength radio waves used for normal television broadcasts.

• The Global Positioning System: a system of satellites, ground monitoring stations, and receivers that determine your exact location at or above Earth’s surface.

  • GPS satellites are owned and operated by the United States Department of Defense, but the microwave signals they send out can be used by anyone.