The Electromagnetic Spectrum and Light

1. The Electromagnetic Spectrum and Light 18.1 Electromagnetic Waves

2. 18.1 Electromagnetic Waves • Electromagnetic wavesare transverse waves consisting of changing electric fields and changing magnetic fields. • Like mechanical waves, electromagnetic waves carry energy from place to place • However, electromagnetic waves differ from mechanical waves in how they are produced and how they travel, so they vary in wavelength and frequency

3. 18.1 How they are produced… • Electromagnetic waves are produced by constantly changing fields. • electric fields exert electric forces on charged particles. • Electric fields are produced by electrically charged particles and by changing magnetic fields • magnetic fieldsproduce magnetic forces. • Magnetic fields are produced by magnets, by changing electric fields, and by vibrating charges

4. 18.1 How they travel • The fields regenerate each other • As the fields regenerate, their energy travels in the form of a wave. • Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter. (Mechanical waves can only travel through matter.) • The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation.

5. The Speed of Electromagnetic Waves • Michelson's Experiment • 1926, the American physicist Albert Michelson (1852–1931) measured the speed of light more accurately than ever before using mirrors.

6. The Speed of Light • light and 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.

7. Wavelength and Frequency • In a vacuum, all electromagnetic waves travel at the same speed. • Electromagnetic waves vary in wavelength and frequency • The speed of an electromagnetic wave is the product of its wavelength and its frequency • Speed = wavelength x frequency v = λ x f • wavelength is inversely proportional to the frequency. As the wavelength increases, the frequency decreases

8. Wavelength and Frequency • In a vacuum, all electromagnetic waves travel at the same speed. • Electromagnetic waves vary in wavelength and frequency • The speed of an electromagnetic wave is the product of its wavelength and its frequency • Speed = wavelength x frequency v = λ x f • wavelength is inversely proportional to the frequency. As the wavelength increases, the frequency decreases

9. Practice Problems • A global positing satellite transmits a radio wave with a wavelength of 19 cm. What is the frequency of the radio wave? (Hint: Convert the wavelength to meters before calculating the frequency.) • The radio waves of a particular AM radio station vibrate 680,000 times per second. What is the wavelength of the wave? • Radio waves that vibrate 160,000,000 times per second are used on some train lines for communications. If radio waves that vibrate half as many times per second were used instead, how would the wavelength change?

10. Problems • What is the wavelength of an FM radio wave in a vacuum if its frequency is 6.92 x 1011 millihertz?

11. Wave or Particle? • Scientists know electromagnetic radiation travels as a wave. • They also have evidence that electromagnetic radiation behaves like a stream of particles • So…Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles.

12. Evidence for the Wave Model • 1801- English Physicist Thomas Young • Shows light behaves like a wave by passing a beam through a series of slits. • Young observed alternating bright and dark bands. This shows interference patterns. (Constructive and Destructive Interference)

13. Evidence for the Particle Model • Photoelectric Effect-the emission of electrons from a metal caused by light striking the metal. • Occurs with high energy (high frequency) blue light • Does not occur with low energy red light. • In 1905, Albert Einstein proposed light (and all EM radiation) contains “packets” of energy called photons.

14. Intensity • The closer you are to a source of light, the brighter the light appears. • Photons travel outward from a light source in all directions. • Intensity- the rate at which a wave’s energy flows through a given unit of area. • Intensity decreases as photons travel further from the source.

15. 18.2 The Electromagnetic Spectrum • William Herschel’s discovery • He used a prism to separate sunlight • Herschel noticed the temperature was lower at the blue end and higher towards the red end. • Temperature was even higher just beyond the red end.

16. Herschel’s Conclusion • Temperature was even higher just beyond the red end. • Herschel concluded there must be invisible radiation beyond the red end of the color band.

17. Electromagnetic Radiation • Today, radiation beyond the red end of the color band is called infrared radiation. • Electromagnetic spectrum- full range of frequencies of electromagnetic radiation. • It includes: • Radio waves • Infrared rays • Visible light • Ultraviolet rays • X-rays • Gamma rays

19. Radio Waves • Radio waves have the longest wavelength and the lowest frequency. • Amplitude modulation (AM)- the amplitude of the wave is varied. Frequency remains the same. • Frequency modulation (FM)- frequency of the wave is varied. Amplitude remains the same.

20. Radio Waves • FM radio signals do not travel as far as AM signals along Earth’s curved surface. • Particles in Earth’s upper atmosphere reflect the lower frequency radio waves of AM better than the higher frequency FM waves. • Radio Waves include: • Microwaves are the shortest wavelength • Radio waves also carry cell phone conversations. • Radar • TV

21. Infrared Rays • Infrared Rays have a higher frequency than radio waves and a lower frequency than red light. • Warmer objects give off more infrared radiation than cooler objects – discovers area of heat differences. • Thermograph- color-coded pictures that show variations in its temperature. • Used to keep food warm at a restaurant and look for people after an earthquake.

22. Visible Light • Visible light is the part of the spectrum that the human eye can see

23. Ultraviolet Rays • UV rays have a higher frequency than violet light. • In moderation, UV rays help your skin produce vitamin D. UV rays are used to kill microorganisms and in plant nurseries. • Excessive exposure causes sunburn, wrinkles, and cancer.

24. X-Rays • X-rays have short wavelengths and high frequencies. • They can penetrate matter that visible light cannot. They are used in medicine, industry, and transportation • Ensures proper seal on cans, identification of trailer contents, suitcase contents, etc. • Too much exposure can kill or damage living tissue

25. Gamma Rays • Gamma rays have the shortest wavelengths and the highest frequencies. • They have the most energy. • Overexposure can be deadly. • Gamma rays are used in radiation therapy to kill cancer cells and mapping the brain. Gamma rays are also used to examine underground pipelines for rusting or cracks.

26. 18.3 Behavior of Light • Materials can be transparent, translucent, or opaque. • Transparent-A material through which you can see clearly. • A transparent material transmits light which means it allows most of the light that strikes it to pass through it. • Translucent-scatters some light • You can see through the material, but the objects you see through it do not look clear or distinct. • Opaque-either absorbs or reflects all of the light that strikes it. • It does not allow any light to pass through. • The order of light transmitting capabilities is: • opaque→translucent→transparent

27. Interactions of Light • 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.

28. Reflection • Image- a copy of an object formed by reflected or refracted waves of light. • When light reflects from a smooth surface, you see a clear, sharp image. • When light reflects from a rough surface, you see a blurred reflected image or no image at all.

29. Reflection • Regular Reflection- occurs when parallel light waves strike a surface and reflect all in the same direction. • regular reflection happens when light hits a smooth, polished surface. • Diffuse Reflection- occurs when parallel light waves strike a rough, uneven surface and reflect in many different directions.

30. Refraction • A light wave can refract, or bend, when it passes at an angle from one medium to another. • Underwater objects look closer and larger. • Refraction can also sometimes cause a mirage. • Mirage- a false or distorted image.

31. Polarization • Polarized Light- light with waves that vibrate in only one plane. • Polarized filters block waves with electric fields vibrating in one direction (or plane). • Ex: polarized sunglasses block vibrating light in one plane

32. Scattering • Light is redirected as it passes through a medium. • A scattering effect reddens the sun at sunset and sunrise. • Small particles scatter shorter-wavelength blue light more often. • When the sun is high in the sky, the blue light is scattered in all directions more than the other colors which is why the sky appears blue.

33. 18.4 Color • As white light passes through a prism, shorter wavelengths refract more than longer wavelengths, and the colors separate. • Dispersion- The process in which white light separates into colors. • Newton’s prism experiment showed that white sunlight is made up of all colors of the spectrum

34. The Color of Objects • The color of any object depends on what the object is made of and on the color of light that strikes the object. • Red paint mostly reflects red light. Most of the other colors of white light are absorbed. • Objects look black when all light is absorbed.

35. Mixing Colors of Light • Primary Colors of Light • Three specific colors that can be combined to create all possible colors. Primary colors are red, green, and blue. • Secondary Colors of Light • Cyan, yellow, and magenta • Each secondary color is a combination of two primary colors.

36. Primary colors of pigments • Cyan, yellow, and magenta • Are the same as the secondary colors of light • Combine in equal amounts to form black

37. Mixing Colors of Light • Any two colors of light that combine to form white light are complementary colors of light (ex. Blue and yellow). • Complementary colors of pigments combine to form black. • Pigments are Cyan, yellow, and magenta • .

38. 18.5 Sources of Light • Define luminous. • Objects that give off their own light. • List several common light sources. • Incandescent, fluorescent, laser, neon, tungsten-halogen and sodium vapor bulbs • What is incandescent? • The light produced when an object gest hot enough to glow. • Describe an incandescent light bulb. • The electrons flow through the filament, and the filament gets hot and emits light

39. 18.5 • What is fluorescence? • A material that absorbs light at one wavelength and then emits light at a longer one. • Describe a fluorescent light bulb. • They emit light by causing a phosphor to steadily emit protons. • Why do schools and offices use fluorescent light bulbs? • Efficiency (last 10x longer) and cost

40. 18.5 • What is a laser? • A device that generates a beam of coherent light • How is a laser light produced? • It is emitted when excited atoms of a solid, liquid or gas emit photons. • What are lasers used for? • To cut through metals, make computer chips, surgeons use lasers to cut/repair damaged tissue, to carry information through optical fibers (fiberoptics) • How do neon lights emit light? • They emit light as electrons move through a gas or a mixture of gases inside glass tubing.

41. 18.5 Sources of Light • Do all neon lights contain the element Ne? • No • What other elements are used in neon lights? • Ar, He, Kr – each kind produces its own distinctive color (it is not the color of the glass, it is the color of the gas) • Describe how sodium-vapor lights work. • 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 to a gas. • What are sodium vapor lights used for? • Street lights, parking lots (Ne, Ar it changes the color)

42. 18.5 • Describe how tungsten-halogen lights work? • Electrons flow through a tungsten filament. The filament gets hot and emits light. • What is the advantage of using tungsten-halogen lights? • They last longer than incandescent lights • Why is quartz used instead of glass? • Quartz has a higher melting point