Waves
The Nature of Waves
What is a mechanical wave?
 A wave is a repeating disturbance or movement that
transfers energy through matter or space
 Each wave carries energy
 Requires matter to travel through  MEDIUM
 Created when a source of energy causes a vibration
to travel through a medium
Types of Mechanical Waves
• Transverse wave = a wave that causes the medium to
vibrate at right angles to the direction in which the wave
travels.
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Line: Rest position
Crest = the highest point of the wave above the rest position
Trough = the lowest point of the wave below the rest position
Ex: S waves in earthquakes
Types of Mechanical Waves
• Longitudinal wave = a wave in which the vibration of the
medium is parallel to the direction in which the wave
travels
• Compression = an area where particles in a medium are spaced
closer together
• Rarefaction = an area where the particles in a medium are spread
out
• Ex: springs, primary (P) waves in earthquakes
Wavelength (λ)
= the distance between a point on one wave and the same
point in the next cycle of the wave
– For a transverse wave: measured between adjacent
crests or adjacent troughs
Wavelength(λ)
• For a longitudinal wave: measured between
adjacent compressions or rarefactions
Amplitude
= the maximum displacement of a medium from its rest position
– For a transverse wave: the distance from the rest position to a crest
or trough
• The more energy a wave has, the greater the amplitude!
Amplitude
– For a longitudinal wave: maximum displacement of a point from its
rest position
• The more energy a wave has, the more the medium will be
compressed!
Frequency & Period
• Period = the time required for one cycle, a complete
motion that returns to its starting point
• Frequency = the number of complete cycles in a given
time; for a wave, the # of wave cycles that pass a point
in a given time
– Unit: hertz (Hz) = cycles per second
– A wave’s frequency equals the frequency of the vibrating source
producing the wave
Mathematically, the period is the reciprocal of the
frequency and vice versa. In equation form, this is
expressed as follows.
Velocity
= the speed of a wave
– Velocity = wavelength x frequency
– Units: meters/second (when wavelength is in meters
& frequency in hertz)
– For waves travelling at a given speed, wavelength is
inversely proportional to frequency.
– Velocity is usually constant, unless the wave enters a
new medium or if variables such as
pressure/temperature change.
Phase
= describes how far to the left or right a wave shifts
Interference
= occurs when 2 or more waves overlap & combine
together
• Constructive Interference = occurs when two or
more waves combine and their displacements add
together; increases amplitude
• Destructive Interference= occurs when two or more
waves combine & their displacements are
subtracted from each other; decreases amplitude
Reflection
= occurs when a wave bounces off a surface that it
cannot pass through
Reflection
• When the wave pulse hits a fixed boundary (a
wall) the wave is reflected back.
**Incoming waves are
known as incident
waves.
A pulse reaching the end of a medium becomes inverted
whenever it either:
(1) reflects off a fixed end,
(2) or is moving in a less dense medium and
reflects off a more dense medium.
Reflection
• If the wave is reaches the boundary for its medium and is
entering a new medium (air to water, or air to object, etc.):
– Part of the wave is reflected back
– Part of the wave is transmitted to the new medium
(1)The transmitted pulse (in the more dense medium) is
traveling slower than the reflected pulse (in the less dense
medium).
(2)The transmitted pulse (in the more dense medium) has a
smaller wavelength than the reflected pulse (in the less
dense medium).
(3)The speed and the wavelength of the reflected pulse are
the same as the speed and the wavelength of the incident
pulse.
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The transmitted pulse (in the less dense medium) is traveling faster than the
reflected pulse (in the more dense medium).
The transmitted pulse (in the less dense medium) has a larger wavelength
than the reflected pulse (in the more dense medium).
The speed and the wavelength of the reflected pulse are the same as the
speed and the wavelength of the incident pulse.
Refraction
= the bending of a wave as it enters a new
medium at an angle.
– When a wave enters a medium at an angle, refraction
occurs because one side of the wave moves more
slowly than the other side.
Diffraction
= the bending of a wave as it moves around an
obstacle or passes through a narrow opening
**The larger the
wavelength is
compared to the
size of an opening
or obstacle, the
more a wave
diffracts.
Electromagnetic Waves
= transverse waves consisting of changing electric
fields and changing magnetic fields
– Produced by a vibrating electric charge and as such,
they consist of both an electric and a magnetic
component
– Carry energy from place to place
– Vary in wavelengths & frequencies
– Can travel through a vacuum or through matter
Electromagnetic Spectrum
= full range of frequencies of electromagnetic waves