WAVE UNIT: SECTION 2
NOTES
Characteristics of Waves
Transverse Waves:


An ideal transverse wave has the shape of a sine curve
and looks like an “S” lying on its side.
The simplest transverse waves have similar shapes no
matter how big they are or what medium they travel
through.
Parts of Transverse Waves



Crests: highest points of a transverse wave
Troughs: lowest points on a transverse wave
Amplitude: greatest distance that particles in a
medium move from their normal position
 Distance

from the rest position to a crest or to a trough.
Wavelength: distance from one crest to the next or
from one trough to the next.
 Basically,
it is the distance between any 2 successive
identical parts of a wave.
Diagram of Transverse Waves
crest
crest
trough
trough
Parts of Longitudinal Waves




The particles move back & forth (parallel) instead
of up & down (perpendicular)
Compressions: crowded areas in a longitudinal
wave
Rarefactions: stretched out areas in a longitudinal
wave
Wavelength: distance between 2 compressions or
between 2 rarefactions
Diagram of a longitudinal wave:
Wavelength


In equations, wavelength is represented by the
Greek letter lambda, λ
Because wavelength is a distance measurement, it is
expressed in the SI unit meters.
Amplitude and Wavelength: Energy

The larger the amplitude of a wave, the more
energy it carries.
 Example:
Waves of destructive earthquakes have
greater amplitudes, and therefore more energy, than
minor earthquakes.

The shorter the wavelength of a wave, the more
energy it carries.
Period



Period: the time it takes one full wavelength of a
wave to pass a certain point.
In equations, the period is represented by the
symbol T.
Because period is a time measurement, it is
expressed in the SI unit seconds (s).
Frequency



Frequency: the number of wavelengths that pass a
point in a given time interval
The symbol for frequency is f.
SI Unit: Hertz (Hz)
 Named
after Heinrich Hertz, the scientist who
demonstrated existence of electromagnetic waves in
1888.
 Hertz units measure the number of vibrations per
second.
 We can hear sounds in the range from 20 Hz to 20,000
Hz.
Frequency-Period Equation:

Wave Speed

Wave Speed Example

The average wavelength in a series of ocean waves
is 15.0 m. A wave crest arrives at the shore on
average every 10.0 s, so the frequency is 0.100
Hz. What is the average speed of the waves?
xλ
 v= 0.100 Hz x 15.0 m
 v= 1.50 m/s
 v=f
Wave Speed and Medium

Speed of a wave depends on the medium:
 Sound
travels through air at 340 m/s (about 770 miles
per hour)
 Since sound travels fast in air, you don’t notice a time
delay in most situations.

Sound travels three to four times faster in water
than in air.
 If
you swim with your head underwater, you can hear
certain sounds very clearly.
 Dolphins use sound waves to communicate with one
another over long distances underwater.
Wave Speed and Medium


Sound waves travel even faster in solids than in air or
water.
 Sound waves have speeds 15 to 20 times as fast in
rock or metal as in air.
Although the speed of a wave depends on the medium,
the speed in a given medium is constant; it does not
depend on the frequency of the wave.
 No matter how fast you create waves on a rope, they
still travel at the same speed; it just increases the
frequency and decreases the wavelength.
Wave Speed and Phases of Matter



Wave speed in gases: a molecule must pass through
a lot of empty space before it bumps into another
molecule; therefore, waves don’t travel as fast in
gases as they do in liquids and solids
Wave speed in liquids: particles are closer together
& free to slide past one another & waves move
faster
Wave speed in solids: particles are packed very
tight together, so vibrations occur very rapidly, so
waves travel very fast
Speed of Light



Speed of light in empty space: 3.0 x 108 m/s
(671,000,000 mi/h) (186,000 mi/s)
The speed of light is a constant that is represented
by the lowercase letter “c”.
Light travels slower when it has to pass through a
medium like air or water.
The Doppler Effect

Have you ever been to a racetrack and noticed how
the sound differs as the cars pass around the track?
 The
motion between the source of waves and the
observer creates a change in observed frequency. In
the case of sound waves, motion creates a change in
pitch.
The Doppler Effect

The pitch, how high or low a sound is, is determined
by the frequency of the waves.
A
high-pitched sound is caused by sound waves of a
higher frequency.
The Doppler Effect

A.) When an object is
not moving, the
frequency of the
waves is the same at
all locations.

B.) When an object is
moving, the sound waves
are closer in front and
farther behind. The
person in front hears a
higher-pitched sound.
The Doppler Effect

So, the Doppler Effect is: an observed change in the
frequency of a wave when the source or observer is
moving.
The Doppler Effect