Light & the Electromagnetic Spectrum Electromagnetic Waves Electromagnetic waves > transverse waves consisting of changing electric & magnetic fields; carry energy from place to place; differ from mechanical waves in how they are produced and how they travel Production of EM waves = produced by constantly changing electric fields (resulting from electrically charged particles) & magnetic fields (resulting from either magnets, changing electric fields and/or vibrating charges); are produced when an electric charge vibrates or accelerates and are at right angles to each other. If you forgot about the homework due today: textbook page 542 data analysis questions, I'll collect them tomorrow along with binder pages 114 115 Travel of EM waves = EM waves don t need a medium because the electric and magnetic fields regenerate each other; EM waves can travel through a vacuum (empty space) or through matter Electromagnetic radiation = the transfer of energy by EM waves through matter or space 1
Speed of light = 299,792,458 m/s To make things easier: speed of light = 3.00 x 10 8 m/s (note that this is for light traveling through a vacuum) Albert Michelson was the first to measure the speed of light accurately in 1926 by setting up a system of light beams & mirrors on 2 mountains about 50 miles apart. It would take light.02 seconds to go from New York City to San Francisco! ALL electromagnetic waves travel at the same speed (3.00 x 10 8 m/s) but EM waves vary in wavelength and frequency. Speed = wavelength x frequency There is evidence that electromagnetic radiation behaves like a wave and also like a stream of particles so we say it is BOTH! Evidence for the wave model > Thomas Young used a series of slits to prove that light exhibits wave interference 2
Only after light passes through the card with 2 slits and you now have 2 waves do you see the pattern of constructive & destructive interference which means light functions as a wave! Evidence for the particle model > Einstein proposed that light and all EM radiation consists of packets of energy called photons; because photons contain energy they must contain mass but it is extremely small; the photon s energy is proportional to the frequency of the light wave being emitted (blue light photons would have greater energy than red light photons because blue light has a higher frequency than red light.) This was proven by the photoelectric effect > the emission of electrons from a metal caused by light striking the metal. Note that the red light didn t have enough energy to cause electron emission. 3
Intensity > the rate at which a wave s energy flows through a given unit of area; similar to brightness; the further you are from the light source, the smaller the intensity because the photons spread over a larger area. Sound waves contain a certain amount of energy. If this energy is spread over a larger area, the intensity of the waves will be less than over a small area. The wave nature of light makes this true for light also > intensity decreases as distance from the light source increases. Section 18.2 The Electromagnetic Spectrum William Herschel used thermometers to measure the temperatures of each color of visible light & discovered that the temperature was highest at the red end (the red end is most concentrated) & lowest at the blue end. He then measured the temperature above the red end, where no light appeared & discovered it to be even higher!! There must be invisible radiation beyond the visible light spectrum! Electromagnetic spectrum = the full range of frequencies of electromagnetic radiation; includes radio waves, infrared rays, visible light, ultraviolet rays, X rays and gamma rays with each category having a range of wavelengths and frequencies. Remember that all portions of the EM spectrum travel at the same speed 3.00 x 10 8 m/s. 4
Possible Back to the Future Topic Ideas: Renewable resources tidal, geothermal... hydrogen bombs IPOD nightvision pagers choclear implants printers CAT scans lasers iris scanners brown noise drones in military Wii Fuel cells smart cars hybrid cars cell phones plasma TV digital TV HD TV Solar vulture aircraft Hubble space exploration... irradiated foods electronic air cleaners MP3 players CD players Lasix Digital cameras palm pilots blackberrys Itouch Iphone computers (be specific) amplifiers copiers automatic sensors UPC scanners Mag Lev Trains microwaves 1. Radio waves > longest wavelengths (1 mm to thousands of km), lowest frequencies (300,000 MHz or less); used in radio & TV technology, microwave ovens and radar A) Radio use = music and voices are changed into electronic signals that are coded onto radio waves and sent out through the air. Coding is done in 2 ways: 1. Amplitude modulation = amplitude of the radio wave is varied (AM stations); uses frequencies between 535 1605 khz 2. Frequency modulation = frequency of the radio wave is varied (FM stations); these waves don t travel as far because particles in earth s upper atmosphere reflect the lower frequency AM wave better; uses frequencies between 88 108 MHz 5
B) Television use = both audio and video signals are coded onto the radio waves; these signals are received by the antenna and sent to the TV set; because antenna can be affected by weather and location, cable and satellite were developed which don t require an antenna. Complete the data analysis questions on How Long Does an Antenna Need to Be on p. 542. C) Microwaves = shortest wavelength of radio waves; wavelength of 1-1000 mm; frequencies of 300-300,000 MHz; can cook and reheat food by transferring energy to the water or fat molecules in food from the microwaves; can only penetrate food a few cm. so the rest of heating must be done by conduction (which is why you have to let it sit!); also carry cell phone conversations 6
D) Radar = Radio Detection And Ranging; uses a radio transmitter to send out short bursts of radio waves which reflect off objects and bounce back to where they came from; the time and shape of the returning waves is interpreted by a receiver. The faster the object is moving towards the radar, the more compressed the waves are and the higher the frequency (the Doppler effect again!). 2) Infrared Rays --> between radio waves and red light; wavelengths from 10-3 - 10-9 m; used as a source of heat and to detect areas of heat differences; can t be seen, but your skin can feel its warmth; used to warm food & reptile habitats. Thermogram = uses infrared sensors to detect differences in infrared emission; produces a thermograph - color coded pictures that indicate temperature variations 7
3) Visible light --> light that the human eye can see; each wavelength corresponds to a certain frequency and has a certain color. Visible light is used to see & to communicate with. 4) Ultraviolet rays --> wavelength of 4 x 10-9 to 4 to 10-11 m; higher frequencies than violet light; used in health, medicine and in agriculture; helps body produce Vitamin D; can cause sunburn, wrinkles and skin cancer and can damage eyes; used to disinfect because it can kill microorganisms; also can be used in winter by nurseries to help plants grow. 8
5) X-rays --> very short wavelengths 1.2 x 10-8 to 1 x 10-12 m; have high energy and can penetrate matter that light cannot. Used in medicine, industry and transportation to make pictures of the inside of solid objects (your body, trucks, packages, etc.) 6) Gamma rays --> shortest wavelength in the spectrum of less than 10-12 m; have highest frequencies, most energy, and greatest penetrating ability; exposure to large amounts of gamma rays can be deadly; used to kill cancer cells, make brain scans & as an inspection tool. 9
Section 18.3 - Behavior of Light How light behaves when it strikes an object depends on the material the object is made of: 1. Transparent = transmits light; can see through clearly; glass, air, water can all be transparent 2. Translucent = scatters light; can see through but objects don t appear clear; frosted glass & plastic are translucent 3. Opaque = absorbs or reflects all of the light that strikes it; allows no light to pass through it; most objects are opaque - wood, clothing & rubber are all opaque 10
Light Interactions --> When light strikes a new medium, the light can be reflected, absorbed or transmitted. If the light is transmitted through the medium, several things can happen - it can be refracted, polarized, or scattered. 1. Reflection --> the bouncing back of light waves; can form an image - a copy of the object formed by reflected waves. Regular reflection = occurs when parallel light waves strike a surface and reflect all in the same direction; results when light hits a smooth, polished surface (a mirror). Diffuse reflection = occurs when parallel light waves strike a rough, uneven surface and reflect in many different directions 11
2. Refraction --> the bending of light waves when it passes at an angle from one medium to another; causes objects to look different in size and/or shape; can cause a mirage = a false or distorted image due to difference in light speed due to temperature differences of air 3. Polarization --> light with waves that vibrate in only one plane; normal, unpolarized light vibrates in all directions but by placing a polarizing filter in front of the light, you block out all light not traveling in the same plane as the filter slits allow; polarized sunglasses work this way!! 12
4. Scattering --> light rays are redirected as they pass through a medium. The earth s atmosphere contains many small particles that scatter light with blue light being scattered the most and red light the least. As the sun sets, you are looking at the sun through more of the earth s atmosphere so most of the blue is scattered and you see more red. When the sun is directly overhead, it is passing through much less of the atmosphere so the blue light is scattered all around and you see the sky as blue (even though it is colorless!) Sect. 18.5 - Sources of Light Luminous = able to give off light Common light sources: 1. Incandescent light --> light is produced when the object gets hot enough to glow; usually has a wire filament made of tungsten surrounded by fairly inert gases like nitrogen and argon; when electrons flow through the filament of an incandescent bulb, the filament gets hot and emits light; more heat then light is produced by these types of bulbs 2. Fluorescent light --> uses fluorescence (a material called a phosphor absorbs light at 1 wavelength and then emits light at a longer wavelength) to cause a phosphor to steadily emit photons 13
EX: a fluorescent bulb is a glass tube coated with phosphors that contains mercury vapor inside the tube. When electricity flows through the bulb, the electrons hit atoms of the mercury vapor causing the mercury atoms to emit ultraviolet rays which then strike the phosphor coating which then causes the phosphor atoms to emit visible light Fluorescent bulbs are very energy efficient because they don t get as hot as incandescent bulbs as most of their energy is emitted as light! 3. Laser light --> Light Amplification by Stimulated Emission of Radiation; a laser generates a beam of coherent light (light in which waves have the same wavelength and the crests and troughs are lined up) when excited atoms of a solid, liquid or gas emit photons; coherent light doesn t spread out so its intensity is fairly constant. Lasers can be used to cut through metals, make computer chips, used to cut or repair human tissue, in fiber optics or to measure distances. 4. Neon light --> emits light when electrons move through a gas or a mixture of gases inside glass tubing; because the photons of different gases have different energies, different colors are produced - neon (red), helium (pink), argon & mercury (greenishblue) and krypton (violet) 14
5. Sodium vapor light --> contains a small amount of solid sodium along with a mixture of neon and argon gases as an electric current passes through a sodium-vapor bulb, it ionizes the gas mixture, which warms the mixture up and causes the sodium to change from a solid into a gas and excites the sodium electrons to higher energy levels. When the electrons move back to lower energy levels, the sodium atoms emit light. Are very efficient and emit bright light - which is why they are often used for parking lot lights and street-lights. 6. Tungsten-Halogen light --> similar to incandescent bulbs but has a halogen gas (I, Br, F) instead of inert gases surrounding the tungsten filament filament gets hot & emits light halogen gas reduces wear on the filament, so these bulbs last longer than incandescent bulbs gets so hot that the glass used is made of quartz as regular glass would melt 15
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