Ch 18 The Electromagnetic Spectrum

What are Electromagnetic Waves? Electromagnetic waves are transverse waves consisting of changing electric fields and changing magnetic fields. Electromagnetic waves are transverse waves because the fields are at right angles to the direction in which the wave travels.

What are Electromagnetic Waves continued Transverse waves, longitudinal waves, and surface waves are all types of mechanical waves Like mechanical waves, electromagnetic waves carry energy from place to place, but are produced and travel differently than mechanical waves

How are they produced? Electromagnetic waves are produced by constantly changing electric fields and magnetic fields. 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. Mechanical waves must have a medium to travel through-they cannot travel through empty space The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation.

The Speed of Electromagnetic Waves In 1926, the American physicist Albert Michelson measured the speed of light more accurately than ever before He timed a light beam as it traveled from one mountain to another and back again. The speed of light = 3.00 × 108 m/s The speed of light is abbreviated with the letter “c”

v=λf How do EM waves differ? Electromagnetic waves vary in wavelength and frequency. The speed of an electromagnetic wave is the product of its wavelength and its frequency. v=speed λ=wavelength f=frequency v=λf

Wave or particle? Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles. Scientists know that electromagnetic radiation travels as a wave. Scientists also have evidence that electromagnetic radiation behaves like a stream of particles. EM waves are often described as having “wave-particle duality” because of this

Evidence for the wave model In 1801 Thomas Young discovered that when light waves pass through a slit the waves interfere, or overlap, with each other When waves interfered constructively a bright band of light was seen as the waves added together When waves interfered destructively a dark band of light was seen

Evidence for the particle model In 1905, Albert Einstein proposed that light, and all electromagnetic radiation, consists of packets of energy. These packets of electromagnetic energy are now called photons. The emission of electrons from a metal caused by light striking the metal is called the photoelectric effect.

The particle model continued… Photons travel out from a light source in all directions Near the light source, the photons spread through a small area, so the light is intense. Farther from the source, the photons spread over a larger area.

Light Intensity Intensity is the rate at which a wave’s energy flows through a given unit of area. For example, 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.

18.2 The Electromagnetic Spectrum The electromagnetic spectrum includes radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

Electromagnetic Spectrum continued… 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 Have the lowest frequencies in the spectrum In a radio studio, sound is changed into electronic signals that are coded onto radio waves and then broadcast. Your radio receives the radio signal, decodes it, and changes it back into sound waves you can hear.

Radio Waves continued… FM radio signals do not travel as far as AM signals along Earth’s curved surface. Particles in Earth’s atmosphere reflect the lower- frequency AM radio waves much better than the FM radio waves. The reflection helps transmit AM signals farther. The shortest-wavelength radio waves are microwaves. Radio waves also carry signals for television programming and make radar technology possible

Infrared Rays 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. Search-and-rescue teams use infrared cameras to locate people who are trapped during disasters.

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.

Ultraviolet Ultraviolet rays have applications in health and medicine, and in agriculture. 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 X-rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects. X-rays have high energy and can penetrate matter that light cannot. Your teeth and bones absorb X-rays. X-ray photographs show softer tissue as dark, highly exposed areas. Bones and teeth appear white. Too much exposure to X-rays can kill or damage living tissue.

Gamma Rays 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. 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.

Gamma Rays continued… 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.

18.3 Behavior of Light Materials can be transparent, translucent, or opaque. 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.

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

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 make an object appear to break at the surface of the water.

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. This reflection produces glare. Polarized sunglasses have vertically polarized filters to block the horizontally polarized light. Vertical polarizing filter Horizontal polarizing filter Vertical wave is blocked Horizontal wave is blocked Vertical wave passes through filter.

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. A scattering effect reddens the sun at sunset and sunrise.

18.4 Color As white light passes through a prism, shorter wavelengths refract more than longer wavelengths, and the colors separate. Sunlight is made up of all the colors of the visible spectrum. A prism separates white light into a visible spectrum. This process is called dispersion. A rainbow forms when droplets of water in the air act like prisms.

Colors of Light The color of any object depends on what the object is made of and on the color of light that strikes the object. The primary and secondary colors of light are different than those you remember from art class Primary colors are three specific colors that can be combined in varying amounts to create all possible colors. The primary colors of light are red, green, and blue. Each secondary color of light is a combination of two primary colors. The secondary colors of light are cyan, yellow, and magenta.

Colors of Light The three primary colors of light are red, green, and blue. When any two primary colors combine, a secondary color is formed.

18.5 Sources of Light Six common light sources include incandescent, fluorescent, laser, neon, tungsten-halogen, and sodium-vapor bulbs. Objects that give off their own light are luminous. The sun is luminous, as are all light sources.

Incandescent Light The light produced when an object gets hot enough to glow is incandescent. Incandescent bulbs give off most of their energy as heat, not light.

Florescent Light In fluorescence, a material absorbs light at one wavelength and then emits light at a longer wavelength. Office buildings and schools use mostly fluorescent lights. Fluorescent tubes do not get as hot as incandescent bulbs because they emit most of their energy as light. This means that they use energy very efficiently.

Laser Light Laser light is emitted when excited atoms of a solid, liquid, or gas emit photons. A laser is a device that generates a beam of coherent light, in which waves have the same wavelength and the crests and troughs of the waves are lined up A beam of coherent light doesn’t spread out significantly from its source, so the light has a relatively constant intensity.

Neon Light Neon lights emit light when electrons move through a gas or a mixture of gases inside glass tubing. Many lights called neon lights contain gases other than neon, including helium, argon, and krypton.

Sodium-Vapor Light As electric current passes through a sodium-vapor bulb, it ionizes the gas mixture. The mixture warms up and the heat causes the sodium to change from a solid into a gas. The yellow color of sodium-vapor light makes objects look different than they look in sunlight.

Tungsten-Halogen Light Inside a tungsten-halogen bulb, electrons flow through a tungsten filament. The filament gets hot and emits light. Tungsten-halogen light is produced in much the same way as incandescent light. The halogen gas reduces wear on the filament, so tungsten-halogen bulbs last longer than incandescent bulbs.