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PHY138 – Waves, Lecture 4 The Plan for Today: Sound and Light Medical Applications of Ultrasound Power and Intensity The Doppler Effect

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Reading Assignment Next week’s reading is Knight Chapter 21, Sections 21.1 – There is a pre-class quiz on for this material due on Monday morning. A Problem Set on Chapter 20 is due on Friday at 11:59PM. A Written Team Problem Set is due next Friday, Nov.24 at 5:00 PM. It’s available in PDF Format on the Waves Class Summaries Page.

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Sound and Light Sound is a pressure wave in a gas, liquid or solid. Speed depends on material. Light is one type of electromagnetic wave. In a vacuum, all electromagnetic waves (including light) travel at c = 3×10 8 m/s. In transparent media, light slows down. Index of Refraction is n > 1. This reduces the wavelength, but does not change the frequency!

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A light wave travels through three transparent materials of equal thickness. Rank is order, from the largest to smallest, the indices of refraction n 1, n 2, and n 3. A. n 2 > n 1 > n 3 B. n 3 > n 1 > n 2 C. n 1 > n 2 > n 3 D. n 3 > n 2 > n 1 E. n 1 = n 2 = n 3

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Reflection of Transverse Wave Pulse A pulse traveling to the right on a heavy string attached to a lighter string Speed suddenly increases

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Reflection of Transverse Wave Pulse A pulse traveling to the right on a light string attached to a heavier string Speed suddenly decreases

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Physics of Ultrasound Speed of sound in bone, flesh and blood are all different are all different When the speed of any wave suddenly changes, there is a reflection and transmission Ultrasound images are formed from reflected high frequency sound Image resolution is set by wavelength, λ λ= v / f, so higher frequency yields smaller λ, and better resolution

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Speed of sound in humans TissueSound Speed (m/s) Air350 Fat1450 Brain1540 Blood1570 Bone4080 Muscle1585

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Power and Intensity The Power, P, of any wave source is how much energy per second is radiated as waves [units = Watts] The Intensity, I, is the energy rate per area. This determines how loud (sound) or bright (light) the wave is. I=P/a, where a is an area perpendicular to the wave direction. At a distance r from a spherically symmetric source, the intensity is I=P/(4πr 2 )

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Chapter 20, Problem 34 The sound intensity from a jack hammer breaking concrete is 2 W/m 2 at a distance of 2 m from the point of impact. This is sufficiently loud to cause permanent hearing damage if the operator doesn’t wear ear protection. What is the sound intensity for a person watching from 50 m away?

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Doppler Effect

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Valerie is standing in the middle of the road, as a police car approaches her at a constant speed, v. The siren on the police car emits a “rest frequency” of f 0. Which statement is true? A.The frequency she hears rises steadily as the police car gets closer and closer. B.The frequency she hears steadily decreases as the police car gets closer and closer. C.The frequency she hears does not change as the police car gets closer.

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Valerie is standing still as a police car approaches her at a constant speed, v. Daniel is in his car moving at the same constant speed, v, toward an identical police car which is standing still. Both hear a siren. Which statement is true? A.The frequency Daniel hears is lower than the frequency Valerie hears. B.The frequency Daniel hears is higher than the frequency Valerie hears. C.The frequencies that Daniel and Valerie hear are exactly the same. (Almost correct) Actual Answer! Check eq and 20.39!

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