Chapter 18: The Electromagnetic Spectrum and Light

Chapter 18: The Electromagnetic Spectrum and Light 18.1 Electromagnetic Waves

Electromagnetic waves are produced when an electric charge vibrates or accelerates. Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter.

Electromagnetic wavesare transverse waves consisting of changing electric fields and changing magnetic fields. • Like mechanical waves, they carry energy from place to place. • Electromagnetic waves differ from mechanical waves in how they are produced and how they travel.

How They Are Produced Electromagnetic waves are produced by constantly changing electric fields and magnetic fields. • An electric fieldexerts electric forces on charged particles. • Electric fields are produced by electrically charged particles and by changing magnetic fields. • A magnetic fieldproduces magnetic forces. • Magnetic fields are produced by magnets, by changing electric fields, and by vibrating charges.

Electromagnetic waves are transverse waves because the fields are at right angles to the direction in which the wave travels.

How They Travel Changing electric fields produce changing magnetic fields, and changing magnetic fields produce changing electric fields, so the fields regenerate each other. • Electromagnetic waves do not need a medium. • The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation.

The Speed of Light All electromagnetic waves travel at the same speed when in a vacuum, regardless of the observer’s motion. The speed of light in a vacuum, c, is 3.00 × 108 meters per second.

The speed of an electromagnetic wave is the product of its wavelength and its frequency. • The speed of electromagnetic waves in a vacuum is constant, so the wavelength is inversely proportional to the frequency. • As the wavelength increases, the frequency decreases.

Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles.

The emission of electrons from a metal caused by light striking the metal is called the photoelectric effect. In 1905, Albert Einstein (1879–1955) proposed that light, and all electromagnetic radiation, consists of packets of energy. These packets of electromagnetic energy are now called photons.

Each photon’s energy is proportional to the frequency of the light. Blue light has a higher frequency than red light, so photons of blue light have more energy than photons of red light.

The intensity of light decreases as photons travel farther from the source.

Intensityis the rate at which a wave’s energy flows through a given unit of area. • As waves travel away from the source, they pass through a larger and larger area. • The total energy does not change, so the wave’s intensity decreases.

Intensity The closer you are to a surface when you spray paint it, the smaller the area the paint covers, and the more intense the paint color looks.

Assessment Questions • How are electromagnetic waves different from all mechanical waves? • Electromagnetic waves don’t carry energy. • Electromagnetic waves are invisible. • Electromagnetic waves are longitudinal waves. • Electromagnetic waves can travel through a vacuum.

Assessment Questions • What is the wavelength of a radio wave that has a frequency of 1.5 x 106 Hz? (c = 3.0x108 m/s) • 45 m • 200 m • 450 m • 2 km

Assessment Questions • What is the wavelength of a radio wave that has a frequency of 1.5 x 106 Hz? (c = 3.0x108 m/s) • 45 m • 200 m • 450 m • 2 km wave speed = λf

Assessment Questions • The photoelectric effect is evidence that light behaves like • a wave. • a particle. • both a wave and a particle. • neither a wave nor a particle.

Assessment Questions 4. As photons travel farther from a light source, the intensity of light stays the same. TrueFalse

Chapter 18: The Electromagnetic Spectrum and Light 18.2 The Electromagnetic Spectrum

The full range of frequencies of electromagnetic radiation 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. • Each kind of wave is characterized by a range of wavelengths and frequencies. All of these waves have many useful applications.

Radio waves have the longest wavelengths in the electromagnetic spectrum. Radio waves also have the lowest frequencies.

Microwaves The shortest-wavelength radio waves are called microwaves. Microwaves cook and reheat food. Microwaves also carry cell phone conversations. The process works much like a radio broadcast.

Radar (radio detection and ranging) Radar technology uses a radio transmitter to send out short bursts of radio waves. • They reflect off the objects they encounter and bounce back toward where they came from. • Returning waves are then picked up by a radio receiver.

Infrared rays have higher frequencies than radio waves and lower frequencies than red light. Your skin senses infrared radiation as warmth. Restaurants use infrared lamps to keep foods warm.

Warmer objects give off more infrared radiation than cooler objects. A device called a thermograph uses infrared sensors to create thermograms, color-coded pictures that show variations in temperature.

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.

Ultraviolet rays have shorter wavelengths and higher frequencies • Some exposure to ultraviolet rays helps your skin produce vitamin D, which helps the body absorb calcium from foods. • Excessive exposure can cause sunburn, wrinkles, skin cancer, and eye damage. • Ultraviolet rays are used to kill microorganisms. In winter, plant nurseries use ultraviolet lights to help plants grow.

X-rays have very short wavelengths X-rays have high energy and can penetrate matter that light cannot. Too much exposure to X-rays can kill or damage living tissue.

Gamma rays have the shortest wavelengths in the electromagnetic spectrum. They have the highest frequencies, the most energy, and the greatest penetrating ability of all the electromagnetic waves. Exposure to tiny amounts of gamma rays is tolerable, but overexposure can be deadly.

Radiation treatment using radioactive cobalt-60. • 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.

Assessment Questions • Which waves have the longest wavelength? • radio waves • infrared rays • visible light • ultraviolet rays

Assessment Questions • What type of electromagnetic radiation is used to keep prepared foods warm in a serving area? • ultraviolet rays • infrared rays • X-rays • gamma rays

Chapter 18: The Electromagnetic Spectrum and Light 18.3 Behavior of Light

A transparent material transmits light, which means it allows most of the light that strikes it to pass through it.

A translucent material scatters light. If you can see through a material, but the objects you see through it do not look clear or distinct, then the material is translucent.

An opaque material either absorbs or reflects all of the light that strikes it. Most materials are opaque. An opaque object does not allow any light to pass through it.

When light strikes a new medium, the light can be reflected, absorbed, or transmitted. When light is transmitted, it can be refracted, polarized, or scattered.

Reflection An imageis a copy of an object formed by reflected (or refracted) waves of light. Regular reflectionoccurs when parallel light waves strike a surface and reflect all in the same direction. Diffuse reflectionoccurs when parallel light waves strike a rough, uneven surface and reflect in many different directions.

Law of Reflection • Angle of incidence (angle of incoming light rays) equals the angle of reflection (angle of reflected light rays)

Refraction A light wave can refract, or bend, when it passes at an angle from one medium into another. Refraction makes underwater objects appear closer and larger than they really are.

Refraction can also sometimes cause a mirage, a false or distorted image. • occurs because light travels faster in hot air than in cooler, denser air.

Polarization Light with waves that vibrate in only one plane is polarized light. • Light reflecting from a nonmetallic flat surface, such as a window or the surface of a lake, can become polarized. • Horizontally polarized light reflects more strongly than the rest of the sunlight. This reflection produces glare. • Polarized sunglasses have vertically polarized filters to block the horizontally polarized light.

Scattering In scattering, light is redirected as it passes through a medium. • Most of the particles in the atmosphere are very small. Small particles scatter shorter-wavelength blue light more than light of longer wavelengths. • Blue light is scattered in all directions more than other colors of light, which makes the sky appear blue.

Assessment Questions • How do polarized sunglasses reduce glare? • by scattering light as it passes through the glasses • by providing a smooth surface that light can reflect off • by absorbing all light • by blocking horizontally polarized light

Chapter 18: The Electromagnetic Spectrum and Light