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Light Part 1 – Electromagnetic Spectrum Modeling Light Light can be described using two different models: the wave model or the particle model. Wave Model The wave model describes light as transverse waves that do not require a medium to travel through. Describe a transverse wave. What type of wave does not need a medium to travel? Particle Model Discussion: Observations that could not be explained by the wave model… The particle model assumes that light is contained in small packets called photons. Photons do not have mass, they are more like bundles of energy. Red Light vs. Blue Light Wave Model & Particle Model According to the wave model, red light should have more energy because it has a greater amplitude… so why couldn’t it knock electrons off the metal plate? The Speed of Light During a thunder storm, what would happen first: you hear the thunder or you see the bolt of lightning? In a vacuum, all light travels at the same speed called c : 3 x 108 meters/second Light is the fastest signal in the universe ** You should memorize this number!! All electromagnetic waves travel at this speed. Speed of Light The speed of light depends on the medium that it travels through. What type of medium is in a vacuum? Light travels slower through slower through a medium than it does in a vacuum. Intensity The quantity that measures how bright a light is, or the amount of light that illuminates a surface, is called intensity. On which squares is the light most intense? The electromagnetic spectrum (EM spectrum) consists of light at all possible energies, frequencies, and wavelengths. It includes radio waves, ultraviolet waves, visible waves, x-rays, and more. Our eyes can detect light waves ranging from 400nm (violet light) to 700nm (red light). This is called the visible light spectrum. Remember: ROYGBIV Sunlight Sunlight contains ultraviolet light (UV light) UV light has higher energy and shorter wavelengths than visible light. UV light causes sunburns. Prolonged or repeated exposure may lead to skin cancer. UV Rays are good too… Cause skin cells to manufacture vitamin D for healthy bones and teeth. Used in hospitals for cleanliness Kill bacteria Class Work Real World Applications p.403 X – Rays High frequency electromagnetic waves, beyond the UV part of the EM spectrum. Used in taking pictures of the inside of the body. X – ray Passes through low density objects but is absorbed by high density objects, like bones and teeth. X rays can be harmful to humans because they can kill healthy body cells, or turn them into cancer cells. Protective lead aprons. Infrared (IR) Light Wavelength longer than visible light. IR light from the sun or a lamp warms you. IR light keeps cafeteria food warm. Given off by hot objects as they cool. Thermogram - A special IR film that detects the amount of IR radiation given off. It can identify warm or cold areas of an object or person. Large amounts = Red, Small amounts = Blue. IR sensitive binoculars enable people to “see” objects in fog or complete darkness. Microwaves Radio wavelengths with wavelengths in centimeters. Microwave ovens in the US use microwaves with a wavelength of about 12 cm. Microwaves are reflected by metals, and go through paper and plastic, but are absorbed by water, fat, and sugars. Microwaves are also used to carry telecommunication signals. Microwaves will create large currents of electricity and can be harmful with people wearing heart pacemakers. Radio Waves Waves with the lowers frequencies are radio waves: AM, FM, short-wave, and television. AM vs. FM Radio AM FM Vary amplitude Amplitude constant Frequency constant: Frequency Varies: 535 kilohertz (535,000) - 88 megahertz (88,000,000) - 1605 kilohertz (1,605,000) 107.9 megahertz (107,900,000) Good over long distances AM travels longer distances than FM waves because AM has a longer wavelength Waves Radio Waves Short-wave radios Longer wavelength than radio Reflect even better than radio International broadcasts Television TV antenna or dish receives waves and uses them to make electrical current Radar Radar waves are at the upper limit of the radio wave frequency. Radar systems use reflected radio waves to determine the distance to and from objects. Air traffic controllers and police officers both use radar systems Light Part 2 – Reflection, Refraction, Lenses & Color Electromagnetic Waves All types of electromagnetic waves have the same properties as visible light. They can be reflected and refracted. Reflected: Bounces off an object. Refracted: Bends going from one material to another. Another Light Model A light ray is a model of light that represents light traveling through space in a straight line. Every object reflects some light and absorbs some light. Light rays can be drawn to show how light is reflected off a surface. Rough surfaces reflect light in many directions… sometimes called a diffuse reflection Reflection: The return of light or sound waves from a substance Angle of Incidence = Angle of reflection Refraction The bending of light waves as they pass from one material into another i r Θi = Θi Θr = Θr r i As light enters a denser material it bends toward the normal Index of Refraction The ratio between the speed of light in a vacuum and the speed of light in a particular substance Hydrogen 1.00013 Air 1.00029 Water 1.33 Glass 1.5 to 2.0 Diamond 2.42 ** The higher the index of refraction, the more the light will slow and bend. Lenses A lens is a transparent object, usually made of glass or plastic. The lens is curved so that it changes that direction of light. The amount the lens is curved and the lens material determines how much the light will be refracted. Convex Lens A lens that is thicker at the center than at the edges Glasses for people that are far sighted Telescopes Microscopes Concave Lens A lens thinner at its center than at the edges Glasses for people who are nearsighted Peepholes for doors Some wide angle camera lenses Concave mirror A spherical reflecting surface that constitutes a segment of the interior of a sphere Flashlights Automobile headlights Convex mirror A spherical reflecting surface that constitutes a segment of the exterior of a sphere. Makes objects appear to be further away because objects look smaller when you look at them in your mirror Seeing Colors Different wavelengths of visible light correspond to different colors. Your brain interprets each wavelength of visible light as a certain color. Every object absorbs or reflects visible light waves. The reflected light waves enter your eyes, your brain interprets them as colors. White light is the combination of all visible light. A prism is a transparent block that can separate white light into its component colors, ROYGBIV. Water in the air can act as a prism, separating white sunlight into the colors of the rainbow. Waves Laser A device that produces a beam of highly concentrated light that is all one frequency or color Light, flashlight, or the sun: The light from these sources is usually a mixture of colors at different frequencies and wavelengths Light waves are all jumbled and independent of each other once they leave the source They go everywhere and become scattered Waves Lasers Waves are all the same frequency and wavelength Crests and troughs line up and amplitudes are the same All waves move in the same direction from the source Waves Definition of: laser (Light Amplification by the Stimulated Emission of Radiation) A device that creates a uniform and coherent light that is very different from an ordinary light bulb. Many lasers deliver light in an almost-perfectly parallel beam (collimated) that is very pure, approaching a single wavelength. Laser light can be focused down to a tiny spot as small as a single wavelength. Laser output can be continuous or pulsed and is used in a myriad of applications. Gas lasers are used to cut steel and perform delicate eye surgery, while solid state lasers create the ultra-high-speed, miniscule pulses traveling in optical fibers traversing the backbones of all major communications networks. Light traveling in an optical fiber is impervious to external interference, a constant problem with electrical pulses in copper wire. Waves LASER How Does It Work? • A laser is an optical oscillator, which is made out of a solid, liquid or gas with mirrors at both ends. To make the laser work, the material is excited or "pumped," with light or electricity. The pumping excites the electrons in the atoms, causing them to jump to higher orbits, creating a "population inversion." • A few of the electrons drop back to lower energy levels spontaneously, releasing a photon (quantum of light). The photons stimulate other excited electrons to emit more photons with the same energy and thus the same wavelength as the original. • The light waves build in strength as they pass through the laser medium, and the mirrors at both ends keep reflecting the light back and forth creating a chain reaction and causing the laser to "lase.“ • In simple laser cavities, one mirror has a small transparent area that lets the laser beam out. In semiconductor lasers, both mirrors often transmit a beam, the second one being used for monitoring purposes. Waves Unit # 7 : Waves: Pretest Problems All electromagnetic waves travel at ________ meters/second The speed of sound @ 0 o C. is _________ meters/second The speed of sound @ 30 o C. is _________ meters/second The change in the speed of sound per change of 1 oC. is ____ The speed of sound @ 50 o C. is _______ meters/second . The speed of sound is 380 meters/second . The temperature is _______ o C. In the summer time (speed of sound : 350 meters/second ) , how far could a sound wave travel in a day ? Waves Unit # 7 : Waves: Pretest Problems On a cold winter day (0 o C.) you beep your snowmobile horn .The sound bounces off a cliff and returns to you in 4 seconds . How far away is the cliff ? Three people are playing jump rope . One person shakes the rope 12 times in 3 seconds . The speed of the rope is 20 meters per second . How long is the wavelength ? At 30 o C. a tuning fork has a wavelength of 5 meters . What is the frequency of the sound it produces ? The speed of a wave is 3.0 x 10 8 meters/second . The wavelength is 3 meters . What is the frequency of the wavelength ?
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