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Light and Sound

This lecture covers fundamental aspects of light and sound, focusing on how sound waves and light intensity are measured and perceived. Topics include ultrasonic pulse timing, sound wavelength calculation, and the relationship between intensity and loudness as perceived by human ears. We explore practical problems related to real-world applications, such as ultrasound resolution for imaging and the energy delivery to the eardrum during whispering. The principles are reinforced with examples, promoting a better understanding of wave behavior in different media.

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Light and Sound

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Presentation Transcript

  1. Light and Sound Lecture 35 Reading: 15: 4-6 Problems: 15: 1, 4, 7, 13, 15, 17, 18

  2. Quiz • vSOUND = 343 m/s • The little bat in the ultrasonic ranger sends out a short 40 khz pulse. It bounces of a target 1.7m away. How much time elapses before he hears the reflected echo?

  3. What is the wavelength of a 40kHz sound wave? 60 • 8.6 mm • 17 m • .56m

  4. Ultrasound • Vwater = 1480 m/s • Objects can be imaged to an approximate level of detail of about one wavelength. What frequency is needed to obtain 0.5 mm resolution of an object in a human body?

  5. Ultrasound • Solution

  6. HeNe Laser • The wavelength of light from a Helium-Neon laser is 6328 angstroms. 1 angstrom = 10-10 m • What is the frequency of this light?

  7. Intensity • Intensity • Point source (that is, we are at a distance large compared to the source dimension)

  8. Sound • Increasing sound intensity by a factor of 10 is perceived as an increase in loudness by a factor of 2. (Ask your biology professor to explain this.) • Decibel scale to measure sound intensity level • Reference is lowest intensity you can perceive in an extremely quiet room

  9. Decibel Scale • Define • Measure loudness relative to threshold of hearing

  10. Problem 15:25 • Sound is detected when a sound wave causes the eardrum to vibrate. Typically the diameter of the eardrum is about 8.4mm. How much energy is delivered to your eardrum each second when someone whispers (20dB) a secret in your ear?

  11. 15:25

  12. 15:25 • Power is W = J/s=I*a

  13. 15:27 • The intensity of electromagnetic waves from the sun is 1.4kW/m2 just above the earth’s atmosphere. Eighty percent of this reaches the earth’s surface on a clear day. Model your back as 30cm x 50 cm rectangle. How many joules of solar energy fall on your back as you work on your tan for 1.0 hr?

  14. 15:27 • Intensity at earth’s surface

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