PHY122_L16_forPostingBeforeLecture

PHY122_L16_forPostingBeforeLecture - PHY122 – Physics for...

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Unformatted text preview: PHY122 – Physics for the Life Sciences II Lecture 16 Waves and Interference HW 10 is due Sunday, 6 Nov. at 8:00 pm Make-ups for Labs 3,4,5 MUST be done this week (or else! As you all know since Day 1 of PHY 122.) Note: Clicker Channel 21 11/01/2011 Lecture 16 1 Lecture 16 2 Electromagnetic Spectrum Frequency and wavelength: f λ = c (in vacuo: 299792458 m/s exactly!) The v i s i b l e part of the EM spectrum is only small, from 400 nm < λ < 800 nm – different wavelengths are seen as different COLORS – from short λ to long: violet , blue , green , yellow , orange , red larger λ (lower f ): – INFRARED, followed by – MICROWAVES: 0.1 mm < λ < 1 m , – RADIOWAVES: λ > 1 m: UHF, VHF, MW, LW, VLF, … shorter λ (higher f ): – ULTRAVIOLET: 1 nm < λ < 400 nm , – SOFT/HARD X-RAY: 1 pm < λ < 1 nm , and – GAMMA: λ < 1 pm , where instead of wavelength one usually uses photon e n e r g y in keV or MeV or GeV or TeV (or higher!) to denote the radiation (k=10 3 , M=10 6 , G=10 9 , T=10 12 ) 11/01/2011 11/01/2011 Lecture 16 3 http://www.youtube.com/watch?v=Rbuhdo0AZDU&feature=related transverse and longitudinal waves LiveWeb2 Slide 3 LiveWeb2 http://www.youtube.com/watch?v=Rbuhdo0AZDU&feature=related LiveWeb, 11/1/2011 Lecture 16 4 Coherence and Monochromaticity Monochromatic (= “one color”) waves are – by definition – waves that have a single unique wavelength λ ( ⟹ 1 frequency) – examples: light of a single color ( λ red =700 nm ), radio waves of a single FM station ( λ = c /(91.1 MHz) ), a sound tone ( λ =( 344 m/s)/(1000 Hz) ), etc. Waves will i n t e r f e r e : “waves” will add wherever waves from two or more sources come together: this is the important principle of “SUPERPOSITION” – e.g., sound-pressure amplitudes from two sources add; for waves meeting on a rope: the amplitudes add as function of time; for EM waves: the electric and magnetic field vectors each add everywhere… – interference is typically not noticed because many frequencies are present in typical “wave trains” or “wave packets” – if m o n o c h r o m a t i c waves meet, AND if they are COHERENT (i.e., they oscillate with a stable difference in P H A S E ), a stable interference pattern will emerge 11/01/2011 11/01/2011 Lecture 16 5 http://www.youtube.com/watch?v=P_rK66GFeI4 constructive & destructive interference of transverse slinky wave packets (pulses) LiveWeb3 Slide 5 LiveWeb3 http://www.youtube.com/watch?v=P_rK66GFeI4 LiveWeb, 11/1/2011 When waves meet an “obstacle” …...
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PHY122_L16_forPostingBeforeLecture - PHY122 – Physics for...

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