Electromagnetic Spectrum and Light

1. Electromagnetic Spectrum and Light Chapter 16

2. Electromagnetic (EM) Waves – transverse waves consisting of changing electric fields and changing magnetic fields. They differ from mechanical waves in the way they are produced and how they travel. They are produced by constantly changing fields. Electric Field – exerts electric forces on charged particles. Magnetic Field – produces magnetic forces produced by magnets or by changing electric fields and vibrating charges. Electromagnetic waves are produced when an electric charge vibrates or accelerates.

3. The figure below shows that the electric and magnetic fields are at right angles to each other. This is a transverse wave because the fields are also at right angles to the direction in which the wave travels.

4. Unlike mechanical waves, electromagnetic waves do NOT need a medium. Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter. The transfer of energy by EM Waves traveling through matter is called electromagnetic radiation. Why can’t the electric field wave exist without the magnetic field wave? They both produce each other!

5. Michelson’s Experiment- 1926 – American physicist Albert Michelson measured the speed of light more accurately than ever. He reflected and refracted light off a mountain series of mirrors and lenses and by knowing the differences, and by timing the light, he concluded the light’s speed. Mirror Semi-silvered Mirror He earned the Nobel Prize in physics making him the first American ever to get this award. Mirror

6. Light and all electromagnetic waves travel at the same speed in a vacuum. The speed of light in a vacuum is 3.00 X 108 m/s. Actually, it is 299,792,458 m/s. Even though all EM waves travel at the same speed in a vacuum, they are not all the same. EM Waves vary in wavelength and frequency. As we already know, the speed is a product of its wavelength and frequency. Wave Speed = Wavelength X Frequency A radio station broadcasts a radio wave with a wavelength of 3.0 meters. What is the frequency of the wave? Frequency = Wave Speed / Wavelength Frequency = 3.00 X 108 m/s = 1.0 X 108 Hz 3.0m

7. A global positioning satellite (GPS) transmits a radio wave with a wavelength of 19cm. What is the frequency of the radio wave? Hint: Wavelength will need to be converted to meters. Wavespeed = Wavelength X Frequency Frequency = Wave Speed / Wavelength (3.00 X 108 m/s) / (0.19m) 1.6 X 109 Hz

8. The radio waves of an AM radio station vibrate 680,000 times per second. What is the wavelength? Wave Speed = Wavelength X Frequency Wave speed / Frequency = Wavelength (3.00 X 108 m/s) / (680,000 Hz) = Wavelength (300,000,000 m/s) / (680,000/s) = 440m

9. 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 were used instead, how would the wavelength change? At 160MHz, WL = WS / F (3.00 X 108 m/s) / (160,000,000Hz) = 1.9m At 80MHz, WL = WS/F (3.00 X 108 m/s) / (80,000,000 Hz) = 3.8m 3.8m – 1.9m = 1.9m Therefore, The wavelength would be 1.9m longer at 80MHz than at 160MHz.

10. Wave or Particle???? The fact that light casts a shadow has been used as evidence for both the wave model of light and the particle model of light. Evidence of the Wave Model – English physicist Thomas Young in 1801, showed that light behaves like a wave.

11. Evidence for the Particle Model The emission of electrons from a metal caused by light striking the metal is called the photoelectric effect. When dim blue light hits a metal such as cesium, an electron is emitted. When a brighter blue light is emitted, more electrons are emitted. But red light, no matter how bright, does not cause the emission of electrons in this particular metal. WHY???

12. In 1905, Albert Einstein proposed that light consists of packets of energy that he named photons. Each photon’s energy is proportional to the frequency of the light. The greater the frequency, the more energy each of its photons has. Blue light has a higher frequency than red light so the photons of blue light have more energy than those of red light. Blue light photons have enough energy to cause electrons to be emitted from a cesium surface.

13. Intensity – The rate at which a wave’s energy flows through a given unit of area. The intensity of light decreases as photons travel farther from the source. As the light rays move farther from the source, the lit area becomes larger, but less intense.

14. The waves of a spectrum – How do you investigate something that is invisible??? This was the problem of astronomer William Herschel in 1800. He used a prism to separate the wavelengths present in sunlight and placed thermometers at various places along the color bands. He discovered that the temperature was lower at the blue end, higher at the red end.

15. Herschel wondered if the temperature increased beyond the red end. He concluded that the area just beyond the red are recorded an even higher temperature showing that there must be invisible radiation beyond the red color band. This is now called infrared radiation.

16. The electromagnetic spectrum consists of radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. This diagram shows the electromagnetic spectrum.

17. Radio waves – used in radio and television technologies, as well as in microwave ovens and radar. AM – Amplitude modulation – The amplitude of the wave is varied but the frequency remains the same. AM stations use 535 KHz – 1605 KHz. FM – Frequency modulation – The frequency of the wave is varied but the amplitude remains the same. FM stations use 88MHz – 108MHz.

18. The shortest wavelength radio waves are called microwaves. These have a wavelength from about 1 meter to about 1 millimeter. These can cook food for us. When the water or fat molecules in the food absorb microwaves, the thermal energy of these molecules increase. Microwaves also carry cell phone conversations, data transfer, and high distant tv and radio transmissions.

19. Infrared rays – wavelengths vary from about 1 millimeter to about 750 nanometers or millionth of a millimeter (billionth of a meter). Infrared rays are used as a source of heat and to discover areas of heat differences. Thermographs use infrared sensors to create thermograms which are color-coded pictures that show variations in temperatures. These are used to find places where a building looses heat and problems in the path of electric current. Search and rescue teams use infrared cameras to locate victims.

20. Visible Light – Each wavelength in the visible spectrum corresponds to a specific frequency and has a particular color. People use visible light to see, to help keep safe, and to communicate with one another.

22. Ultraviolet Rays – Has higher frequency than violet light and has applications in health, medicine, and agriculture. In moderation, UV rays help your skin produce Vitamin D which helps the body absorb calcium from foods which produce healthy bones and teeth. Excessive exposure can cause skin cancer, sunburn, and wrinkles. It can also permanently damage your eyes. UV rays are used to kill microorganisms as well as help plants to grow.

23. X-Rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects. They have higher frequencies than ultraviolet rays. X-Rays have high energy and can penetrate objects that light cannot.

24. Gamma Rays – Have highest frequencies and the most energy and the greatest penetrating ability of all electromagnetic waves. Overexposure can be deadly! Gamma rays are used in medical field to kill cancer cells and make pictures of the brain. They are used in industry as an inspection tool.

25. Light and Materials Light can be: Transparent – transmits light which allows most of the light to pass through it. Translucent – scatters light such as frosted glass. You can see through it but the objects are fuzzy or lack detail. Opaque – absorbs or reflects all of the light that strikes it. It does not allow any light to pass through making it so you can’t see through it.

26. When light strikes a new medium, the light can be: Reflected Absorbed Transmitted When light is transmitted, it can be: Refracted Polarized Scattered

27. Regular reflection – occurs when parallel light waves strike a surface and reflect all in the same direction. EX. a calm lake acting as a mirror.

28. Diffuse reflection – occurs when parallel light waves strike a rough uneven surface and reflect in many different directions. EX. a dog.

29. Image – a copy of an object formed by reflected or refracted waves of light. Mirage – a false or distorted image. mirages occur because light travels faster in hot air than in cooler dense air. On a sunny day, air tends to be hotter just above the surface of a road than higher up. Light is gradually refracted as it moves into the layers of hotter air. This causes some of the light to curve rather than being on a direct path to the ground.

30. Polarized light – light with waves that vibrate in only one plane. Polarized filters transmit light waves that vibrate this way.

31. Scattering – light is redirected as it passes through a medium. A scattering of light reddens the sun at sunset and sunrise. The tiny molecules in the Earth’s atmosphere can scatter sunlight causing this.

32. The small particles in the atmosphere scatter shorter wavelength blue light more than light of longer wavelength. By the time the sunlight reaches your eyes, most of the blue and even some of the green light has been scattered by the small particles. Most of what remains are the reds and oranges. When the sun is high in the sky, the light travels a shorter distance through the atmosphere. It scatters blue light in all directions which explains why the sky appears blue on a sunny day even though the air itself has no color.

33. As white light passes through a prism, shorter wavelengths refract more than longer wavelengths, and the colors separate. The process in which white light separates into colors is dispersion.

34. The color of any object depends on what the object is made of and on the color of light that strikes the object. Sunlight contains all of the visible colors but when you look at a yellow car in the sunlight, the yellow paint reflects mostly yellow light. Most of the other colors in white light are absorbed at the surface.

35. Mixing Colors Primary Colors – three basic colors (red, green, blue) that can be combined to produce white light. These colors can combine in varying amounts to produce all possible colors. Secondary Colors – A combination of two primary colors making cyan, yellow, and magenta. If you add a primary color to the proper secondary color, you get white light. Any two colors of light that combine to form white light are complementary colors of light. This is a secondary and primary color that combines to form white. Blue and yellow = white Red and cyan = white Green and magenta = white

37. A pigment is a material that absorbs some colors of light and reflects other colors. Paints, inks, photographs, and dyes get their colors from pigments. The primary colors of pigments are cyan, yellow, and magenta. Color printers use these colors plus black to create almost any color. The secondary colors of pigments are red, green and blue. Any two colors of pigments that combine to make black pigment are called complementary colors of pigments.

39. Sources of Light Objects that give off their own source of light are luminous. EX. Sun, lamps, headlights, flashlights, fire, etc. Common light sources include: incandescent flourescent laser neon tungsten-halogen sodium-vapor bulbs

40. Incandescent Light -The light produced when an object gets hot enough to glow. When electrons flow through the filament of an incandescent bulb, the filament gets hot and emits light. Filament of regular light bulbs are made of tungsten. The bulbs are filled with a mixture of nitrogen and argon gas at a very low pressure. These gases do not react with the filament as oxygen does so the filament doesn’t burn out as fast. Incandescent bulbs give off most of their energy as heat…….. not light!

41. Fluorescent Light During fluorescence, a material absorbs light at one wavelength and emits light at a longer wavelength. A phosphor is a solid material that can emit light by fluorescence. Fluorescent light bulbs emit light by causing a phosphor to steadily emit photons. A fluorescent bulb is a glass tube that contains mercury vapor and a glass coated with phosphors. When current flows through the bulb, small electrodes heat up and emit electrons. The electrons hit the atoms of mercury and emit UV rays. The UV rays strike the phosphor coating and emit visible light. They emit most of their energy as light. One 18w fluorescent = one 75 w incand.

42. Laser Light A laser is a device that generates a beam of coherent light. The word laser stands for light amplification by stimulated emission of radiation. Laser light is emitted when excited atoms of a solid, liquid, or gas emit photons. Light in which waves have the same wavelength, and the crests and troughs are lined up is called coherent light. A beam of coherent light does not spread out from its source.

43. Neon Light Neon lights emit light when electrons move through a gas or a mixture of gases inside glass tubing. Many contain gases other than neon such as helium, argon, and krypton for the different colors they produce.

44. sodium –Vapor lights Sodium-vapor lights contain small amounts of solid sodium plus a mixture of neon and argon gases. As electric current passes through a sodium-vapor bulb, it ionizes the gas mixture. This mixture warms up and the heat causes the sodium to change from a solid into a gas. The sodium atoms emit light. These are very energy efficient and give off a very bright light but are slow to come on. They can also alter the color of the objects below.

45. Tungsten-Halogen Lights Works much like an incandescent but it has a small amount of halogen gas such as iodine, bromine, or fluorine. These bulbs last much longer than incandescent because the halogen gas reduces wear on the filament. The light is made of quartz because the heat generated can melt glass. These make colors “pop” and are used in accent lighting, recessed lights, studios, and concert arenas.