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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 × 10^8 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.
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.
- Determine the frequency of a red laser with a 530 nm wavelength.
- Determine the wavelength of a radio signal at 100.1 MHz.
- Determine the wavelength of a wave with a frequency of 1.2 × 10^17 Hz.
- 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?
- 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.
- 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.
- 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?
- Determine the wavelength and amplitude of the wave below. If the frequency is 400 Hz, determine the speed of the wave.
