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Transcript
Electromagnetic Radiation Lecture 10 - Light and Color What exactly are they? (Giancoli Chapter 22) Electromagnetic Induction Electromagnetic Waves Speed of Light The EM spectrum Light and Color Radio Electromagnetic Induction demonstration James Clerk Maxwell Electromagnetic Waves • What does Maxwell have in his hand? • Why is he one of the 3 greatest scientists of all time? • Who are the others? • What on earth do magnets have to do with light and color? James Clerk Maxwell From his studies on electromagnetic induction, Maxwell realized: “Accelerating Charges give rise to Electromagnetic Waves” Maxwell's equations demonstrate that electricity, magnetism and light are all manifestations of the same phenomenon, namely the electromagnetic field." 22.2 Production of Electromagnetic Waves Maxwell invented the field called “Dimensional Analysis” Realized that the ratio of Electric to Magnetic Field had the dimensions of a velocity (m/s), and this term appeared in an expression for the velocity at which EM waves should travel E=v=c B This is the speed of light in a vacuum. ConcepTest 22.2 The electric field in an EM wave traveling northeast oscillates up and down. In what plane does the magnetic field oscillate? Oscillations 1) In the north-south plane. 2) In the up-down plane. 3) In the NE-SW plane. 4) In the NW-SE plane. 5) In the east-west plane. ConcepTest 22.2 The electric field in an EM wave traveling northeast oscillates up and down. In what plane does the magnetic field oscillate? Oscillations 1) In the north-south plane. 2) In the up-down plane. 3) In the NE-SW plane. 4) In the NW-SE plane. 5) In the east-west plane. The magnetic field oscillates perpendicular to BOTH the electric field and the direction of the wave. Therefore the magnetic field must oscillate in the NW-SE plane. Light as an Electromagnetic Wave and the Electromagnetic Spectrum Light was known to be a wave, and experiments demonstrated that EM waves share all of the same behaviors. The frequency of an electromagnetic wave is related to its wavelength: c = 3x108 m/s This wave-speed equation is the same one that applies to sound, and any other waves. A few years after Maxwell’s breakthrough, Tesla invented (and patented) radio technology (1897) Marconi commercialized it and Wireless communication revolutionized the world within a few short years. (First TransAt: 1901) Measuring the Speed of Light • The speed of light was known to be very large, (Galilio concluded that either light traveled instantaneously, or it was too fast to be measured given human reaction times) • Studies of the orbits of Jupiter’s moons (Ole Romer: 1676) showed that their positions fall successively “behind” where they are supposed to be, as the distance between Earth and Jupiter increases. • The moons are ~40min late when Earth and Jupiter are on opposite sides of the Sun . • Using the familiar laws of gravity and circular motion to compute the distance to Jupiter, the speed of light was found. • Romer’s result was within a few percent of the modern value 22.4 Measuring the Speed of Light • One important measurement, by the Polish-American physicist Michelson in 1925 used a rotating mirror and a light beam bounced between two mountain tops in California. • Over the years, measurements have become more and more precise; now the speed of light is defined to be: If the sun (150x103 km away) were to somehow lose power and go dark, how long would it take us to notice? A. 8 min B. 1 min C instantaneously D. about a year E. 8 seconds Light as an Electromagnetic Wave and the Electromagnetic Spectrum Electromagnetic waves can have any wavelength; we have given different names to different parts of the wavelength spectrum. What type of EM wave could have a wavelenght of 1 meter ? A. Light B. Radio C. X-ray D. Infrared Electromagnetic Spectrum Which Electromagnetic waves travel the fastest? 1. 2. 3. 4. 5. X-rays Red Light Radio waves Microwaves All the same Light and Vision Although Aristotle and other ancient scientists had already written on the nature of light and color vision, it was not until Newton that light was identified as the source of the color sensation. In 1810, Goethe published his comprehensive Theory of Colors. In 1801 Thomas Young proposed his trichromatic theory, based on the observation that any color could be matched with a combination of three lights. This theory was later refined by James Clerk Maxwell and Hermann von Helmholtz. As Helmholtz puts it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's theory of color sensations, like so much else that this marvellous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."[4]" http://lite.bu.edu/vision-flash10/applets/Color/Color/Color.html Wavelength response curves for Human Cone Cells • B G R • We typically think of these cells as having R, G, B wavelength coverage, the actual responses are shown at left, and overlap significantly, probably to give maximum sensitivity and color discrimination in the green. Color perception arises from the ratio of R/G/B counts resulting from the convolution of the incident spectrum with these response curves. Temperature and the EM Spectrum • All everyday objects emit electromagnetic radiation • The part of the spectrum depends on their temperature If you heat something up so that it begins to glow, what color tends to appear first? 1. 2. 3. 4. 5. Blue Green Red Yellow Orange What colors are possible for glowing hot objects? • • • The shape and width of the spectrum result in a certain range of possible colors or hues for incandescent objects. Natural width of the black-body distribution is much greater than the band-pass of the eyes’ “Green” cones, so green stars for example are not possible. This color-space diagram shows the color of a glowing hot object as a function of its temperature 22.5 Energy in EM Waves The energy transported through a unit area per unit time is called the intensity: (22-7) Its average value is given by: Units are Watts per square meter W/m2 For example the “Solar Constant” is ~ 1300 W/m2 Radio, TV, Wireless Communication This figure illustrates the process by which a radio station transmits information. The audio signal is combined with a carrier wave: • Remember those Capacitor “Time Constants” ? • Radio Transmitters contain a special circuit containing a coil and a capacitor, designed to “oscillate” at kHz - MHz frequencies. • This circuit creates the carrier wave. • The audio signal is added by simply connecting the microphone output into the circuit. Amplitude Modulation The mixing of signal and carrier can be done two ways. First, by using the signal to modify the amplitude of the carrier (AM): Note the audio signal is only approximated by the AM envelope. The closer together the carrier “peaks” the more faithfully the signal can be reproduced. ConcepTest 22.4 If a radio transmitter has a vertical antenna, should a receiver’s antenna be vertical or horizontal to obtain the best reception? Radio Antennas 1) vertical 2) horizontal 3) doesn’t matter ConcepTest 22.4 If a radio transmitter has a vertical antenna, should a receiver’s antenna be vertical or horizontal to obtain the best reception? Radio Antennas 1) vertical 2) horizontal 3) doesn’t matter If a wave is sent out from a vertical antenna, the electric field oscillates up and down. Thus, the receiver’s E field antenna should also be vertical so of wave that the arriving electric field can set the charges in motion. E field of wave ConcepTest 22.3 Before the days of cable, televisions often had two antennae on them, one straight, and one circular. Which antenna picked up the magnetic oscillations? TV Antennas 1) the circular one 2) the straight one 3) both equally, they were straight and circular for different reasons. ConcepTest 22.3 Before the days of cable, televisions often had two antennae on them, one straight, and one circular. Which antenna picked up the magnetic oscillations? The varying B field in the loop means the flux is changing and therefore an EMF is induced. TV Antennas 1) the circular one 2) the straight one 3) both equally, they were straight and circular for different reasons. FM - Frequency Modulation Second, by using the signal to modify the frequency of the carrier (FM): FM results in much better signal reproduction, partly because the signal strength is constant, and partly because it is achieved at higher frequencies (more bits of information per second) Why use a carrier wave at all? -Why not just broadcast the raw audio signal as an electromagnetic wave? • To reduce the wavelength for efficient transmission and reception (the optimum antenna size is ½ or ¼ of a wavelength). • A typical audio frequency of 3000 Hz will have a wavelength of 100 km and So would need an effective antenna length of 25 km! (this goes for both reception and transmission) • By comparison, a typical carrier for FM is 100 MHz, with a wavelength of 3m, and could use an antenna only 80 cm long. • The energy Density in very long wavelength wave is so small that interference would be overwhelming. • Long wavelengths do not behave in a very directional way (which for certain very specialized applications is extremely useful) • Digital Radio is even more efficient than FM, it uses a very high frequency, and basically switches the signal on and off rapidly to transmit 1’s and zero’s. Many more stations can occupy the the radio spectrum 22.7 Radio and Television; Wireless Communication At the receiving end, the wave is received, demodulated, amplified, and sent to a loudspeaker: 22.7 Radio and Television; Wireless Communication The receiving antenna is bathed in waves of many frequencies; a tuner is used to select the desired one: Summary of Chapter 22 • Maxwell’s equations are the basic equations of electromagnetism • Electromagnetic waves are produced by accelerating charges; the propagation speed is given by: • The fields are perpendicular to each other and to the direction of propagation. The simplest radio consists of something to rectify the signal (a diode in this case), and a tiny speaker A Diode is a one-way valve for electric current. It isolates the positive side of the radio waveform. Earpiece Diode Diodes today use germanium or silicon (semiconductors) In the past Galena crystals were used, Even rusty razor blades and pencil leads can do the job in a pinch In a Crystal Radio set, where does the power come from to make an audible signal in the earpiece? A. Batteries B. The radio waves themselves C. Magnets in the earpiece Momentum Transfer and Radiation Pressure In addition to carrying energy, electromagnetic waves also carry momentum. This means that a force will be exerted by the wave. The radiation pressure is related to the average intensity. It is a minimum if the wave is fully absorbed: And a maximum if it is fully reflected: ConcepTest 22.5 Heat Insulation Imagine you are an alien from another planet with infrared eyes. What do you see when you look around the room? 1) Bright spots where the bodies are and dark elsewhere. 2) Dark spots where the bodies are and bright elsewhere. 3) The same as what we see, only everything looks red. 4) The same as what we see, except that red is invisible. ConcepTest 22.5 Heat Insulation Imagine you are an alien from another planet with infrared eyes. What do you see when you look around the room? 1) Bright spots where the bodies are and dark elsewhere. 2) Dark spots where the bodies are and bright elsewhere. 3) The same as what we see, only everything looks red. 4) The same as what we see, except that red is invisible. Bodies are sources of heat and therefore emit infrared radiation. An alien with an instrument to detect infrared would see these sources as bright spots. Infrared photo of a building to check the heat insulation – where are the problem spots in this case?