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

2. Quiz v SOUND = 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? 1.8.6 mm 2.17 m 3..56m 60

4. Ultrasound V water = 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 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. Power is W = J/s=I*a

13. 15:27 The intensity of electromagnetic waves from the sun is 1.4kW/m 2 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