1. PHY138 – Waves, Lecture 4 Today’s overview
Sound and Light
Power and Intensity
The Doppler Effect
Medical Applications of Ultrasound

2. Reading Assignment
Next week’s reading is Knight Chapter 21, Sections 21.1 – There is a pre-class quiz on for this material due on Monday morning.
A Problem Set on Chapter 20 is due on Friday at 5:00PM.

3. 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!

4. Reflection of Transverse Wave Pulse
(Chapter 21 concept.)
A pulse traveling to the right on a heavy string attached to a lighter string
Speed suddenly increases

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

6. 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

7. Speed of sound in humans
Tissue
Sound Speed (m/s)
Air
350
Fat
1450
Brain
1540
Blood
1570
Bone
4080
Muscle
1585

8. 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 small source, the intensity is I=P/(4πr2)

9. Doppler Effect