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Light and Sound

Light and Sound. Lecture 35 Reading : 15: 4-6 Problems: 15: 1, 4, 7, 13, 15, 17, 18. Quiz. v SOUND = 343 m/s The little bat in the ultrasonic ranger sends out a short 40 k hz pulse. It bounces of a target 1.7m away. How much time elapses before he hears the reflected echo?.

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Light and Sound

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  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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