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Light and Sound Lecture 35 Reading: 15: 4-6 Problems: 15: 1, 4, 7, 13, 15, 17, 18 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? 33% 33% .5 6m 8. 6 m m 33% 17 1. 8.6 mm 2. 17 m 3. .56m 60 m What is the wavelength of a 40kHz sound wave? 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? Ultrasound • Solution f v 4 0.5 m m 5. *10 m f v 1480 m / s 5 *10 4 m 2.96 *10 H z 3 M H z 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? Intensity • Intensity I P W atts a m 2 • Point source (that is, we are at a distance large compared to the source dimension) I (r ) PSO U R C E 4 r 2 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 I 0 1.0 *10 12 W m 2 Decibel Scale • Define (10 d B ) log 10 I I 0 • Measure loudness relative to threshold of hearing 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? 15:25 (10 d B ) log 10 I I0 I 20 (10 d B ) * log I0 I log 2 I0 10 I log I0 I 10 100 2 I0 I 100 I 0 100 *10 12 W m 2 10 10 W m 2 15:25 • Power is W = J/s=I*a P Ia 10 10 W m 5.5 *10 15 W * (.0042 m ) 2 2 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? 15:27 • Intensity at earth’s surface I S 0.80 *1.4 kW m 2 1.12 kW m 2 1120 W m 2 P Ia 1120 * (.30 * .50) 168W 168 J s E P ( t ) 168 J s (3600 s hr ) 6 *10 J 5