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PHY138 – Waves, Lecture 4 The Plan for Today:

PHY138 – Waves, Lecture 4 The Plan for Today:. Sound and Light Medical Applications of Ultrasound Power and Intensity The Doppler Effect. Meet & Greet. Friday, Nov. 16 th @ 4:00pm MSB 3153. Talk to medical students from UofT to learn about their experiences in applying to medical school.

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PHY138 – Waves, Lecture 4 The Plan for Today:

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

  2. Meet & Greet Friday, Nov. 16th @ 4:00pm MSB 3153 Talk to medical students from UofT to learn about their experiences in applying to medical school. Food and drinks will be served. A free Kaplan MCAT course will be raffled off! *****$5 membership feewill be charged for non-members. This will provide access to all of the upcoming events for 2007-2008.***** A clipboard is being passed around so that students can add their email address to the Pre-med society email list. They will receive emails from the society regarding any upcoming events and important announcements.

  3. Reading Assignment • Next week’s reading is Knight Chapter 21, Sections 21.1 – 21.8. There is a pre-class quiz on www.masteringphysics.com for this material due on Monday morning. • A www.masteringphysics.comProblem 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.

  4. 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 = 3×108 m/s. • In transparent media, light slows down. Index of Refraction is n > 1. • This reduces the wavelength, but does not change the frequency!

  5. A light wave travels through three transparent materials of equal thickness. Rank is order, from the largest to smallest, the indices of refraction n1, n2, and n3. A.n2 > n1 > n3B.n3 > n1 > n2C.n1 > n2 > n3 D.n3 > n2 > n1E.n1 = n2 = n3

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

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

  8. Physics of Ultrasound • Speed of sound in bone, flesh and blood 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

  9. Speed of sound in humans

  10. 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/(4πr2)

  11. Chapter 20, Problem 34 • The sound intensity from a jack hammer breaking concrete is 2 W/m2 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?

  12. Doppler Effect

  13. Which statement is true? 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 f0. • The frequency she hears rises steadily as the police car gets closer and closer. • The frequency she hears steadily decreases as the police car gets closer and closer. • The frequency she hears does not change as the police car gets closer.

  14. Which statement is true? 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. Actual Answer! Check eq.20.38 and 20.39! • The frequency Daniel hears is lower than the frequency Valerie hears. • The frequency Daniel hears is higher than the frequency Valerie hears. • The frequencies that Daniel and Valerie hear are exactly the same. (Almost correct)

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