Chapter 14: Waves

What’s disturbing you?

Wave Properties

Waves carry energy through matter.

The matter can move with the wave, or at

right angles to it.

Newton’s laws and conservation laws

govern the behavior of waves as well as

particles.

All waves transmit energy. Some we see

and some we cannot, like sound and light.

What are Waves?Rhythmic disturbances that carry energy

through matter or space are called waves.

Some waves require a medium to travel

through. These are called Mechanical waves.

Water, air, ropes, and springs can “carry”

energy through them.

Media can be solids, liquids, or gases. Some

are good carriers and some not so good…

Two types of waves

There are two different fundamental types of

waves:

Transverse Waves- the energy moves at

right angles to the direction of the medium. Water, light, waves on a rope, electromagnetic

waves, most waves move this way

Longitudinal Waves- the energy moves

parallel with the direction of the medium. Sound & slinky compressions are the only examples

of a longitudinal or compression wave. Most liquids &

gases transmit energy this way.

Identifying Waves

Measuring Waves

We can describe waves in several aspects:

Speed- Δd/Δt, speed depends on medium

Amplitude- maximum displacement from

rest or equilibrium. Greater amplitude is

caused by more work, thus more energy

(not more speed)

For waves of the same speed, the rate at

which energy is transferred is proportional

to the square of the amplitude. Double

amplitude transfers 4x as much

energy/sec.

Measuring waves cont’d

Wavelength (λ)- low points are troughs, high

points are crests, shortest distance b/t 2 identical

points on a wave is one wavelength (m).

Period (T)- the time it takes for a wave to make

one complete cycle (oscillation). (s)

Frequency (f)- the number of cycles per second

(Hz).

fT

1

Tf

1

Wave Speed

Both period and frequency of a wave

depend only on the wave source; not speed

or medium.

Wavelength depends on both frequency

and wave speed. Speed of a wave is

wavelength divided by period, so

Wave speed is frequency times

wavelength.

fvT

v

Example Problem

A sound wave produced by a clock chime is

heard 515 m away, 1.5 s later.

a. What is the speed of sound of the

clock’s chime in air?

b. The sound wave has a frequency of

436 Hz. What is its period?

c. What is its wavelength?

Problem Solved

Given: d = 515m, t=1.5s, f=436 Hz

v = d / t, T = 1/f

a. v = 515m / 1.5s = 343m/s

b. T= 1/f = 1/436Hz, T=2.29x10-3s

c. λ = v / f = (343m/s) / (436Hz) = 0.787m

Your turn to Practice 14.1

Do Practice Problems # 2-7 pg 335

Do Ch 14 Review pg 344 #s 3 & 4

Do Ch 14 Review pg 345 #s 6, 7, 11, 24,

25, 26, 28, 30

Do Ch 14 Review pg 346 #s 33 & 36

Wave Behavior When a wave reaches a boundary, some of the

wave reflects back into the original medium, and some of the wave is transmitted into a new medium.

The amount of reflection or refraction depends on rigidity of the medium.

Waves at Boundaries

Remember the speed of a wave through a

medium depends on the properties of the

medium, not wave amplitude or frequency.

In air, temperature affects speed.

In water, depth affects speed.

In solids, rigidity affects speed.

Waves striking boundaries may be

returned to their medium (reflected), or

passed through to the next (transmitted).

Boundaries cont’d

An incoming wave is called an INCIDENT

WAVE.

If an incident wave strikes a barrier and is

transmitted, the pulse remains upward.

If an incident wave strikes a barrier and is

reflected, the pulse returns to the original

medium and can be inverted.

Reflected waves can lose amplitude and

transfer energy to the barrier. The wave’s

speed does not really decrease.

Superposition

Unlike particles of matter, 2 or more waves

can exist in the same space at the same

time.

The medium will be displaced an algebraic

equivalent to the sum of the individual

displacements.

This is called interference and can

increase (Constructive) or decrease

(Destructive) the amplitude of the new

wave.

Wave Interference Interference can produce

points of zero displacement

called nodes.

Points of maximum

displacement are called

antinodes.

Standing waves appear to

stand still due to

interference at just the right

frequency.

Waves in two-dimensions

Ray diagrams help model the movement

of waves in 2 dimensions.

A line perpendicular to the barrier is the

“normal”

The angle an incident ray makes with the

normal will be equal to the angle a

reflected ray makes with the normal

This is the law of reflection and holds true

for all types of waves.

Reflection and Refraction

Waves that return to their medium reflect from a surface following the law of reflection.

Waves that pass through to another medium refract or bend due to a change in speed in the new medium.

Diffraction

Waves may bend AROUND a

barrier they encounter.

Bending around the edges

without changing media is

called diffraction.

Diffraction also occurs when

waves meet a small obstacle.

They can bend around it and

fill in behind it.

Diffraction and Interference

For multiple openings,

waves bend and

interfere with each

other creating

constructive (large

waves or bright

areas) and

destructive (reduces

waves or dark areas)

interference patterns.

Your turn to Practice

Please do Ch 14 Review p 345 #s 20, 22,

23, 45, & 47