Chapter 18 The Electromagnetic Spectrum and Light 18.1 Electromagnetic Waves 18.2 The Electromagnetic Spectrum 18.3 Behavior of Light

18.1 Electromagnetic Waves What are Electromagnetic Waves? Def.-transverse waves consisting of changing electric fields and changing magnetic fields Differ from mechanical waves in how they are produced and how they travel

18.1 Electromagnetic Waves What are Electromagnetic Waves?: How They Are Produced Produced by constantly changing fields Electric field-a field in a region of space that exerts electric forces on charged particles Electric fields are produced by electrically charged particles and by changing magnetic fields

18.1 Electromagnetic Waves What are Electromagnetic Waves?: How They Are Produced Magnetic Field-a field in a region of space that exerts magnetic forces Magnetic fields are produced by magnets, changing electric fields, and by vibrating charges Key Concept: Electromagnetic waves are produced when an electric charge vibrates or accelerates.

18.1 Electromagnetic Waves What are Electromagnetic Waves?: How They Travel Changing electric fields create changing magnetic fields and vice versa. They regenerate each other; as they regenerate, their energy travels in the form of a wave Electromagnetic waves do not need a medium (mechanical waves do)

Electromagnetic Waves Figure 2 Electromagnetic Waves

18.1 Electromagnetic Waves What are Electromagnetic Waves?: How They Travel Key Concept: Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter. Electromagnetic radiation-the transfer of energy by electromagnetic waves traveling through matter or across a space

18.1 Electromagnetic Waves The Speed of Electromagnetic Waves: Michelson’s Experiment and The Speed of Light Light travels faster than sound (lightening occurs before thunder) Albert Michelson (1852-1931)-physicist-calculated the speed of light

18.1 Electromagnetic Waves The Speed of Electromagnetic Waves: Michelson’s Experiment 8-sided rotating mirror on one mountain; 1 mirror on another mountain opposite of 1st mountain Shined light at one face of the rotating mirror; light was reflected to stationary mirror on opposite mountain. Knowing how fast mirror rotated and how far the light traveled from mountain to mountain and back again=speed of light calculated

18.1 Electromagnetic Waves The Speed of Electromagnetic Waves: Michelson’s Experiment and The Speed of Light Light and all electromagnetic waves travel at the same speed when in a vacuum regardless of observer’s motion Key Concept: The speed of light in a vacuum, c, is 3.00 X 108 m/s

18.1 Electromagnetic Waves Wavelength and Frequency Key Concept: Electromagnetic waves vary in wavelength and frequency. Wavelength is inversely proportional to the frequency b/c the speed of electromagnetic waves in a vacuum is constant. Speed=wavelength x frequency

Section 18.1

18.1 Electromagnetic Waves Wave or Particle? Electromagnetic radiation can also behave as a stream of particles. Newton was first to propose a particle explanation. Evidence: light travels in a straight line and it casts as shadow

18.1 Electromagnetic Waves Wave or Particle? Is light a wave or a particle? It is both. Key Concept: Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles.

18.1 Electromagnetic Waves Wave or Particle?: Evidence for the Wave Model Thomas Young-physicist-showed that light behaves like a wave. Passed beam of light through a single slit then a double slit (bright and dark bands) Bands showed that light had produced an interference pattern (bright=constructive; dark=destructive)

Interference (Wave Model of Light) Figure 5 Interference (Wave Model of Light)

18.1 Electromagnetic Waves Wave or Particle? Evidence for the Particle Model (Einstein) Blue light causes electrons to be emitted; red light does not Photoelectric effect-the emission of electrons from a metal caused by light striking the metal Einstein proposed that light and all electromagnetic radiation consists of packets of energy (photons)

18.1 Electromagnetic Waves Wave or Particle?: Evidence for the Particle Model The greater the frequency of and electromagnetic wave, the more energy each of its photons has. Blue light=higher frequency (photons have more energy so electrons emitted); red light=lower frequency (photons have less energy so no emission of electrons)

Photoelectric Effect (Particle Model of Light) Figure 6 Photoelectric Effect (Particle Model of Light)

18.1 Electromagnetic Waves Intensity The closer to light you are, the brighter it is. The further away you are, the dimmer the light. Photons from light travel outward in all directions. In a small area (near the light) light is more intense.

18.1 Electromagnetic Waves Intensity Def.-the rate at which a wave’s energy flows through a given unit (ie. brightness) Key Concept: The intensity of light decreases as photons travel farther from the source.

18.2 The Electromagnetic Spectrum The Waves of the Spectrum William Herschel (1800)-used a prism to separate the wavelengths present in sunlight Colors: red, orange, yellow, green, blue, and violet Each color’s temperature is different; temp. is higher in area beyond red color band (no color present); there is invisible radiation beyond the red end of the color band (infrared radiation)

18.2 The Electromagnetic Spectrum The Waves of the Spectrum Electromagnetic spectrum-the full range of frequencies of electromagnetic radiation Key Concept: The electromagnetic spectrum includes radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

The Electromagnetic Spectrum Figure 9 The Electromagnetic Spectrum

18.2 The Electromagnetic Spectrum Radio Waves Have the longest wavelengths in the spectrum; have lowest frequency Key Concept: Radio waves are used in radio and television technologies, as well as in microwave ovens and radar.

18.2 The Electromagnetic Spectrum Radio Waves: Radio Two ways radio stations code and transmit information on radio waves Both are based on a wave of constant frequency and amplitude. Pg. 541 Amplified modulation (AM)-the amplitude of the wave is varied; frequency remains the same

18.2 The Electromagnetic Spectrum Radio Waves: Radio Frequency modulation (FM)-the frequency of the wave is varied; the amplitude remains the same AM signals travel farther than FM signals

18.2 The Electromagnetic Spectrum Radio Waves: Television Radio waves carry signals for TV programming. The process is like transmitting radio signals BUT radio waves carry information for pictures as well as for sound

18.2 The Electromagnetic Spectrum Radio Waves: Microwaves Def.-the shortest-wavelength radio waves Cook and reheat food (water or fat molecules absorb microwaves=increase in thermal energy of the molecules) Also carry cell phone conversations.

18.2 The Electromagnetic Spectrum Radio Waves: Radar Radar technology uses a radio transmitter to send out short bursts of radio waves Waves reflect off objects encountered, bounce back to where they came from (picked up and interpreted by radio receiver)

18.2 The Electromagnetic Spectrum Infrared Rays Have higher frequencies than radio waves; lower frequencies than red light Key Concept: Infrared rays are used as a source of heat and to discover areas of heat differences. Can’t be seen; can be felt (warmth) Thermograms-color-coded pictures that show variations in temperature

18.2 The Electromagnetic Spectrum Visible Light The visible part of the electromagnetic spectrum Key Concept: People use visible light to see, to help keep them safe, and to communicate with one another.

Figure 13

18.2 The Electromagnetic Spectrum Ultraviolet Rays Ultraviolet radiation has higher frequencies than violet light. Key Concept: Ultraviolet rays have applications in health and medicine, and in agriculture. Help skin produce vitamin D; kill microorganisms; help plants grow Excessive exposure: sunburn, wrinkles, and eventually skin cancer

18.2 The Electromagnetic Spectrum X-Rays Have very short wavelengths; have higher frequencies than UV rays Key Concept: X-rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects. Teeth and bones absorb x-rays (white); tissue (dark); too much exposure can kill or damage living tissue

18.2 The Electromagnetic Spectrum Gamma Rays Have the shortest wavelengths in the electromagnetic spectrum; have the highest frequencies Have the most energy and greatest penetrating ability of all the electromagnetic waves

18.2 The Electromagnetic Spectrum Gamma Rays Overexposure to gamma rays can be deadly. Key Concept: 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.

18.3 Behavior of Light Light and Materials Nothing is visible without light. How light behaves when it strikes an object depends on many factors (this includes the material the object is made of) Key Concept: Materials can be transparent, translucent, or opaque.

18.3 Behavior of Light Light and Materials Transparent-material transmits light (allows most of the light that strikes it to pass through) Ex. Windows, water Translucent-material scatters light (can see through the material, but objects seen through it don’t look clear or distinct) Ex. Frosted glass, some soaps

18.3 Behavior of Light Light and Materials Most materials are opaque. Opaque-material either absorbs or reflects all of the light that strikes it Does not allow any light to pass through Ex. Wood, metal

18.3 Behavior of Light Interactions of Light When light encounters matter, some or all of the energy in the light can be transferred to the matter. Light can affect matter; matter can affect light. Key Concept: 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.

Interactions of Light: Reflection 18.3 Behavior of Light Interactions of Light: Reflection Image-a copy of an object formed by reflected (or refracted) waves of light **[image in a mirror] When light reflects from a smooth surface=clear, sharp image When light reflects from a rough surface=blurred image or no image Ex. Calm water vs. “moving” water

Interactions of Light: Reflection 18.3 Behavior of Light Interactions of Light: Reflection Regular reflection- occurs when parallel light waves strike a surface and reflect all in the same direction Diffuse reflection-occurs when parallel light waves strikes a rough, uneven surface, and reflect in many different directions

Interactions of Light: Refraction 18.3 Behavior of Light Interactions of Light: Refraction A light wave can refract, or bend, when it passes at an angle from one medium into another. Ex. Underwater objects appear closer and larger; Objects appear to break at the surface of water Can cause a mirage-a false or distorted image Mirage-occurs because light travels faster in hot air than in cooler denser air; refracts more as it enters hotter and hotter air=light follows curved path to the ground; light looks like it was reflected from water

Interactions of Light: Polarization 18.3 Behavior of Light Interactions of Light: Polarization Polarized light-light with waves that vibrate in only one plane Polarizing filters transmit this type of light Unpolarized light vibrates in all directions. Vertical polarizing filter-stops light vibrating on a horizontal plane (polarized sunglasses block horizontal polarized light from the sun *glare b/c horizontal light reflects more strongly*) Horizontal polarizing filter-blocks waves vibrating on a vertical plane

Figure 20 Polarization

Interactions of Light: Scattering 18.3 Behavior of Light Interactions of Light: Scattering Earth’s atmosphere has many molecules and other tiny particles= sunlight can be scattered Def.-light is redirected as it passes through a medium Small particles in the air scatter shorter-wavelength blue light more than light of longer wavelengths.

Interactions of Light: Scattering 18.3 Behavior of Light Interactions of Light: Scattering By the time sunlight reaches your eyes, most of the blue and some green and yellow have been scattered. What’s left that we see? Longer wavelengths of light, orange and red (sunset/sunrise) Why is the sky blue? The sun scatters blue light in all directions more than other colors of light. Sky appears blue although air is colorless.