Wave Propagation
A wave is a disturbance that travels through a medium from one location to another. a wave is the motion
of a disturbance.
Waves can go through things. Waves go through water, light can pass through glass, and sound can pass
through walls. The material the wave passes through is called a medium.
A single disturbance passing through a medium is called a pulse. When the disturbances are repeated in a
periodic manner, the pulses generate a continuous wave.
Longitudinal waves (also called compression waves – a slinky pushed and pulled) – moves parallel (same
direction) to the wave motion. Sound waves are longitudinal waves – the sound moves forward and the
oscillations move back and forth.
Sound travels as a longitudinal wave.
 The wave travels in the same direction and the vibration creating the wave.
 Sound requires matter to travel. Sound cannot travel in the vacuum of space. The faster the
vibration can be passed from particle to particle, the faster sound travels. Sound travels fast in dense
materials and air that is hot.
 v = 331 + 0.6T, so at 20 oC, the speed of sound is 343 m/s.
 As sound passes from one medium to another, the speed changes, the wavelength changes, but the
frequency remains constant.
Transverse waves (slinky moving side to side or up and down) – moves perpendicular to the direction of
the wave. Ocean waves are transvers waves, moving forward, but the oscillating up and down.
Light travels and a transverse wave.
 The wave travels perpendicularly to the vibration creating the wave.
 Light does not require matter to travel. Light can travel in the vacuum of space.
 Light travels at 3 × 108 m/s in a vacuum.
 Lights slows down when it travels through material with greater indices of refraction.
 As light travels from one medium to another, the speed changes, the wavelength changes, but he
frequency remains constant.
Different waves have different speeds and different wavelengths.
The speed of travel of a wave depends upon the properties of the medium in which is travels. Sound travels
faster in air that is dense or hot. The more dense the material, the faster the particles can collide sending the
disturbance forward. The hotter the air, the faster the particles in the medium are moving.
The speed of a wave in any medium is given by the equation
v = fλ
where f is the frequency in hertz, HZ and λ is the wavelength in meters.
The speed of the wave on a string depends upon the tension in the string and the density of the string. The
equation for the speed in a string is
or
Where T is tension and μ is the mass density of the string in kg/m.
Barrier Interaction
Fixed Barrier
Free Barrier
When a wave meets a barrier, the wave is
reflected.
If the barrier the wave meets is fixed and
cannot move, then the reflected wave
undergoes an 180o phase change.
If the barrie the wave meets is free to
move, then the wave does not undergo a
phase change.
A similar phenomenon occurs when a wave passes between mediums, such as the rope in the diagrams. If
the wave passes from a low density cord into a high density cord, the transmitted wave slows down and has
a shorter wavelength. The reflected wave undergoes an 180o phase change.
When a wave passes from a cord with high density into one with low density, the wavelength gets longer
and the wave travels forward. The reflected wave does not undergo a phase change.
1. Determine the frequency of a red laser with a 530 nm wavelength.
2. Determine the wavelength of a radio signal at 100.1 MHz.
3. Determine the wavelength of a wave with a frequency of 1.2 × 1017 Hz.
4. You are standing 500 m away from a large cliff in a valley when you clap your hands. You hear the echo
3 seconds later. What is the speed of sound in the air?
5. The tension in a 2.5 m long rope with a velocity of 0.20 m/s is 4.0 N. Determine the mass of the rope.
6. A 25 m long string with a mass of 0.50 kg has a tension of 20.0 N. Determine the speed of a wave on
this string.
7. A wave whose wavelength is 0.40 m is traveling down a 500 m long wire whose mass is 20 kg. The
tension in the wire is 1000 N. What is the speed and frequency of the wave?
8. Determine the wavelength and amplitude of the wave below. If the frequency is 400 Hz, determine the
speed of the wave.