PHY138 – Waves, Lecture 6Today’s overview

Waves in the Electric and Magnetic Fields The Electromagnetic Spectrum The Special Properties of Laser Light

Reading Assignment Please read the following from Serway

and Jewett before class on Monday: Chapter 25, Sections 25.1 through 25.4.

A www.masteringphysics.com assignment is due this Friday at 5:00 PM. It is based on Chapter 14 material: interference, standing waves and harmonics.

Note from last day’s lecture: For a standing wave of sound in an air

column, the pressure nodes correspond to displacement antinodes. Conversely, the pressure antinodes correspond to displacement nodes.

It is the pressure variations which cause a microphone or eardrum to vibrate. Therefore, the standing wave sounds loudest near the pressure antinodes, or displacement nodes.

(this can be discovered in the “Standing Waves and Acoustic Resonance” Lab)

Pair 1 Pair 2

The traces above show beats that occur when two different pairs of waves are added. For which of the two is the difference in frequency of the original waves greater?

1. Pair 12. Pair 23. The frequency difference is the same for both.4. Need more information.

Pair 1 Pair 2

The traces above show beats that occur when two different pairs of waves are added. For which of the two is the sum in frequency of the original waves greater?

1. Pair 12. Pair 23. The frequency sum is the same for both.4. Need more information.

Which of these thingies are not the same?

HeatMicrowaves

Sound

Light

Radio

InfraredUltraviolet

X-rays

Electromagnetic Waves

Everything you need to know about Electricity and Magnetism (for now…)E

B

= Electric Field, which produces a force on charged objects. Units are Newtons/Coulomb, where “Coulomb” is a measure of electric charge.

= Magnetic Field, which causes compasses to point, and produces a force on moving charged objects. Units are called “Tesla”.

Results of Maxwell’s Equations:

2

2

22

2 1

t

B

cx

B

2

2

22

2 1

t

E

cx

E

The Wave Equation for

Electric Field.

The Wave Equation for Magnetic Field.

c is a new constant of the universe: c = 2.998×108 metres/second.

Results of Maxwell’s Equations:

cB

E

The relating equations between Electric and Magnetic fields in an electromagnetic wave.

c is a constant of the universe: c = 2.998×108 metres/second.

BE

The Electromagnetic Spectrum

Radio

Microwaves

Infrared Waves

Visible Light

Ultraviolet Light

X-rays

Gamma Rays

higher frequency

longer wavelength

act more like waves

act more like particles

“Lasers” Light Amplified by Stimulated Emission

Radiation Eye surgery: corneal transplants, vision

correction Heart surgery Laser imaging for diagnosis Laser dentistry