Transcript WAVES

Waves
What are waves?
• Wave: a disturbance that transfers energy from place
to place. (Energy from a wave of water can lift a boat.)
• Medium: –the state of matter (solid, liquid, or gas)
through which a wave can travel. Waves that travel
through matter (air, water) are called Mechanical
Waves. The density of the medium can affect how fast
the wave can travel.
• Waves are created when a source of energy causes a
medium to vibrate. The vibration is a back and forth or
up and down motion of the matter.
Types of Waves:
• Waves are classified according to how they move.
Transverse Waves - (means across)
• moves the medium at right angles (perpendicular) to the
direction in which the waves are traveling.
• As the wave moves in one direction, the particles of the
medium move across the direction of the wave.
Crest (high part of the wave)
normal
wave direction
trough (low part of the wave)
Transverse Wave
Longitudinal Waves (also called Compressional Waves)
• wave that moves particles of the medium parallel to the
direction of the wave. (coils of a spring move back and
forth in the same direction as the wave travels)
rarefactions-(part where coil is spread out)
compressions-part where coil is closed
As compressions and rarefactions travel along the spring,
coils moves forward, then back to their resting position
Longitudinal Wave
SURFACE WAVES
• A surface wave is a combination of transverse
and longitudinal waves.
• occur at the surface between 2 mediums.
• As they pass through water, the water moves up
and down and back and forth. This combination
makes a water particle move in a circle
Air
(gas)
Water
(liquid)
Properties of Waves
Amplitude
• maximum distance the particles of the medium
move away from their rest positions.
• a measure of how much a particle in the medium
moves when it is disturbed by the wave.
• amplitude of a water wave is the maximum
distance a water particle moves above or below the
surface level of calm water.
• Amplitude is a direct measure of the amount of
energy of the wave.
Transverse Waves that have high amplitude,
have high crests and low troughs.
Longitudinal Waves that have high amplitude,
have crowded compressions and spread out
rarefactions.
Wavelength is the distance between two
corresponding parts of a wave.
Wavelength
Wavelength
Frequency
• the number of complete waves or cycles that pass
a given point in a certain amount of time.
• can also be described as the number of
vibrations/second
• measured in units called Hertz (Hz)
• a wave or vibration that occurs every second has a
frequency of 1Hz.
Wave Speed
• The speed of a wave is how far the wave travels in one
unit of time.
• Wave Speed = Wavelength (x) Frequency
• V = λ •f
SAMPLE PROBLEM:
• The speed of a wave on a rope is 50 cm/s and its
wavelength is 10cm. What is its frequency?
f= V
λ
f = 50 cm/s
10 cm
f = 5/s or 5 Hz
V
λ
:
x f
Let’s review the types
of waves and the four
wave properties.
1. Wavelength (lambda)– distance
from a point on a wave to the
same corresponding point on the
next wave.
2. Frequency (f)- number of waves
that pass a point in one second
(expressed in Hz).
Wavelength has an inverse
relationship to wave frequency.
3. Wave velocity (speed)
depends on the type of wave and
medium.
1) Sound is faster in more
dense media and in higher temps.
2) Light is slower in more dense
media, but faster in a vacuum.
4. Amplitude – its size is
related to the amount of
energy carried by the wave.
1) Transverse - how high
above or how low below the
Normal (crest & troughs).
2) Longitudinal- how dense
the medium is at the
compressions & rarefactions.
Interaction of Waves
• 1. Reflection
• When a wave hits a surface through which it cannot
pass, it bounces back.
• The incoming wave bounces back as an outgoing wave.
• The angle of incidence is equal to the angle of reflection
Wave Interactions
Reflection - the bouncing back
of a wave.
1) Sound echoes
2) Light images in
mirrors
3) Law of reflection
i = r
• 2. Refraction
• When a wave moves from one medium to
another medium at an angle, it changes speed as
it enters the second medium, which causes it to
bend.
• (The bending is caused by one side of the wave
entering the medium before the other.)
Refraction - the bending
of a wave caused by a
change in speed as the
wave moves from one
medium to another.
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• 3. Diffraction
• When a wave passes a barrier or moves
through a hole in a barrier, it bends and
spreads out.
Diffraction - the bending of a
wave around the edge of an
object.
1) Water waves
bending around
islands
2) Water waves
passing through a
slit and spreading
out
Diffraction
Diffraction depends on the
size of the obstacle or
opening compared to the
wavelength of the wave.
Less occurs if wavelength is
smaller than the object.
More occurs if wavelength is
larger than the object.
• 4. Interference
• when 2 or more waves meet or hit each other.
• Constructive - 2 waves combine to form a wave
with higher amplitude
• Destructive -2 waves combine to form a wave
with lower amplitude
Interference two or more
waves
overlapping
to form a new
wave.
Constructive (in phase)
Sound waves that interfere
constructively are louder.
Destructive (out of phase)
Sound waves that interfere
destructively are not as loud.
• 5. Standing Waves
• Standing waves are waves that appear to stand
still. They are produced when 2 waves are
interfering with each other.
• Waves are traveling in both directions.
• Node - a point established in a standing wave
where 2 waves collide to produce an amplitude
of zero
• Antinode - a point established in a standing
wave where 2 waves combine to form an
amplitude greater than zero. (max. energy)
Standing Wave
Antinode
Incoming
wave
Reflected
wave
Node
Wall
Standing Wave
Standing wave - a wave pattern
that occurs when two waves
equal in wavelength and
frequency meet from opposite
directions and continuously
interfere with each other.
node
antinode
• Resonance
• This happens when the vibration traveling
through an object matches the object’s normal
frequency.
• If an object is not very flexible, resonance can
shatter an object.
Resonance - the ability of an
object to vibrate by absorbing
energy at its natural
frequency.
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