Chapter 15: Electromagnetic Radiation

Section 1: What are electromagnetic waves?

• Waves in Space

  • No matter where you are, you are surrounded by electromagnetic waves.
  • Waves are produced by something that vibrates, and they transmit energy from one place to another.
  • Water waves and sound waves require matter to move through. Energy is transferred from one particle to the next as the wave travels through the matter.
  • Electromagnetic Waves: made by vibrating electric charges and can travel through space where matter is not present.

• Electric and Magnetic Fields

  • Fields enable magnets and charges to exert forces at a distance. These fields extend throughout space.
  • 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, the net flow of electrons in one direction, is always surrounded by a magnetic field.
  • Electrons moving in a wire are surrounded by a magnetic field.
  • A changing magnetic field creates a changing electric field.

• Making Electromagnetic Waves

  • Electromagnetic waves also are produced when something vibrates—an electric charge that moves back and forth.
  • Because the electric and magnetic fields vibrate at right angles to the direction the wave travels, an electromagnetic wave is a transverse wave.
  • 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 transferred by an electromagnetic wave
  • All electromagnetic waves travel at 300,000 km/s in the vacuum of space.
  • The speed of light is nature’s speed limit—nothing travels faster than the speed of light.
  • The wavelength of an electromagnetic wave is the distance between the crests of the vibrating electric field or magnetic field.

• Waves and Particles

  • Photon: 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.
  • The entire range of electromagnetic wave frequencies is known as the electromagnetic spectrum
  • Electromagnetic waves are described by different names, depending on their frequencies and wavelengths.
  • Visible Light: The electromagnetic waves that humans can detect with their eyes

• 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 30 cm
  • 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.
  • Magnetic resonance imaging technology uses radio waves as an alternative to X-ray imaging.

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

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

• Visible Light

  • Visible light is 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.

• Ultraviolet Waves: electromagnetic waves with wave- lengths 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 and medical supplies.

  • Police detectives sometimes use fluorescent powder to show fingerprints when solving crimes.

  • 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 decrease in ozone is caused by the

    presence of certain chemicals, such as CFCs, high in Earth’s atmosphere.

• 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 10 billionths of a meter and 10 trillionths of a meter.
  • Bones are more dense than surrounding tissues and absorb more X rays. The image of a bone on an X ray is the shadow cast by the bone as X rays pass through the soft tissue.
  • Gamma Rays: Electromagnetic waves with wavelengths shorter than about 10 trillionths of a meter
  • Both X rays and gamma rays are used in a technique called radiation therapy to kill diseased cells in the human body.

• Radio Transmission

  • Radio waves exert a force on the electrons in an antenna, causing the electrons to vibrate.
  • Carrier Wave: The specific frequency of the electromagnetic wave that a radio station is assigned
  • An AM radio station broadcasts information by varying the amplitude of 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).
  • Electronic signals are transmitted by FM radio stations by varying the frequency of the carrier wave

• Television

  • Cathode-Ray Tube: a sealed vacuum tube in which one or more beams of electrons are produced.
  • Cathode-ray tubes 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.
  • The antenna at the top of a microwave tower receives

  signals from nearby cell phones.

  • 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.

• Communications 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 make a call on a mobile telephone, the telephone transmits radio waves directly to a satellite. The satellite relays the signal to a ground station, and the call is passed on to the telephone network.
  • Television satellites use microwaves rather than the longer-wavelength radio waves used for normal television broadcasts.

• Global Positioning System (GPS): a system of satellites, ground monitoring stations, and receivers that determine a person’s or object’s exact location at or above Earth’s surface.

  • A GPS receiver uses signals from orbiting satellites to determine the receiver’s location.
  • 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.