Waves
Waves Transfer Energy
Waves Have Measurable Properties
Waves Behave in Predictable Ways
Definitions
wave - a disturbance that transfers energy from one place to another
Key Concepts
A wave is a disturbance
Waves can transfer energy from one place to another. Waves can transfer energy over distance without moving matter the entire distance, ex:an ocean wave can travel many kilometers without the water itself moving many kilometers. The water moves up and down-a motion known as a disturbance
Forces and Waves
Forces are required to change the motion of an object but can also start a disturbance, sending a wave through a material.
ex: Rope Wave - suppose a rope is tied to a doorknob, you apply one force to the rope by flicking it upward and an opposite force when you snap it back down. This sends a wave through the rope. Both forces are required to start a wave.
ex: Water Wave - forces are also required to start a wave in water. Suppose there is a calm pool of water. When you apply a force to the water by dipping your finger into it, the water rushes back after you remove your finger. The force of your finger and the force of the water rushing back send waves across the pool.
ex: Earthquake Wave - an earthquake is a sudden release of energy that has built up in rock as a result of the surrounding rock pushing and pulling in it. When these two forces cause the rock to suddenly break away and move, the energy is transferred as a wave through the ground
Materials and Waves
medium - any substance that a wave moves through. Water is the medium for an ocean wave; the ground is the medium for an earthquake wave; the rope is the medium for the rope wave.
a rope tied to a doorknob, water, and the ground are all materials through which waves move. Waves can transfer energy through matter as well as empty space
Waves that transfer energy through matter are known as mechanical waves
Energy and Waves
the waves caused by an earthquake are good examples of energy transfer. The disturbed ground shakes from side to side and up and down as the waves move through it. The ground does not travel kilometers away from where it began; it is the energy that travels in a wave. It is kinetic energy, that is transferred. Ocean waves are another good example of energy transfer. They travel to the shore, one after another. Instead of piling up all the water on the shore, the waves transfer energy.
Waves can be classified by how they move
Transverse Waves
In a transversal wave, the direction in which the wave travels is perpendicular, or at right angles, to the direction of the disturbance. Transverse means 'across' or 'crosswise.' The wave itself moves crosswise as compare with the vertical motion of the medium. ex: Rope Wave, Water Wave
transverse wave - the direction in which the wave travels is perpendicular to to the direction of the disturbance
Longitudinal Waves
In a longitudinal wave, the wave travels in the same direction as the disturbance. It can be started in a spring by moving it forward and backward. The forward and backward motion is the disturbance. They are sometimes called compressional waves because the bunched up area is known as a compression. ex: sound wave - imagine a bell ringing. The clapper inside the bell strikes the side and makes it vibrate. The air molecules, in turn, set more air molecules into motion. A sound wave pushes forward. The vibrations of the air are in the same direction as the movement of the wave.
longitudinal wave (compressional wave) - the wave travels in the same direction as the disturbance
Key Concepts
Definitions
Waves have amplitude, wavelength, and frequency
Height is a property of all waves-from ripples in a glass of water to gigantic waves at surfing beaches; measurable, the Speed of water is another property that can be measured-by finding the time it takes for one wave peak to travel a set distance. Other properties of a wave that can be measured include the time between waves and the length of a single wave. Scientists use the terms AMPLITUDE, WAVELENGTH, and FREQUENCY to refer to some commonly measured properties of waves
How Frequency and Wavelength are Related - when frequency increases more wave crests pass a fixed point each second. So as frequency increases, wavelength decreases. Vice versa is also true.
crest - highest point (peak) of a wave
trough - the lowest point (valley) of a wave
AMPLITUDE - the distance a medium moves above or below its position at rest; for a transverse wave, it is the distance between a line through the middle of a wave and a crest or a trough. In an ocean wave, amplitude measures how far the wave rises above, or dips below, its original position, or rest position
WAVELENGTH - the distance from one crest or trough to the next
FREQUENCY - the number of waves passing a fixed point in a certain amount of time, how often a wave occurs; it is often measured by counting the number of crests or troughs that pass by a given point in one second
Graphing Wave Properties - The graph of a transverse wave looks like a wave itself. Unlike transverse waves, longitudinal waves look different from their graphs. In a longitudinal wave in a spring, the coils of the spring get closer and then farther apart as the wave moves through them. The shape of the graph looks like the shape of a transverse wave. The wavelength on a longitudinal wave is the distance from one compression to the next. The amplitude of a longitudinal wave measures how compressed the medium gets. Just as in a transverse wave, frequency in a longitudinal wave is the number of waves passing a fixed point in a certain amount of time.
Equations
Wave speed can be measured
To find the speed of a wave, you can time how long it takes for a wave to get from one point to another or you can calculate the speed. The speed of any wave can be calculated when both the frequency and the wavelength are known.
Speed of a wave: Speed = wavelength*frequency, S=λf, you can think of the wavelength as "meters per wave"
Definitions
Key Concepts
Waves interact with materials
Reflection
When waves encounter a new medium such as water waves encountering a wall, they cannot travel through the wall. Instead, the waves bounce off the wall. This bouncing back after striking a barrier is called reflection. Waves transfer energy and the wall applies an equal and opposite force on the water, sending the wave back in another direction. Sound and light wave reflect too. Sound waves reflecting off a canyon produce an echo. Light waves reflecting off smooth metal behind glass let you see an image of yourself in the mirror.
Reflection - the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated
Refraction
Refraction - the bending of a wave as it encounters a new medium at an angle other than 90 degrees
Sometimes, a wave does not bounce back after encountering a new medium. Instead, the wave continues moving forward. When a wave enters a new medium at an angle, it bends, or refracts. This occurs because waves travel at different speeds in different mediums. Because the wave enters the new medium at an angle, one side of the wave enters the new medium before the rest of the wave. When this side speeds up or slows down before the other side, it causes the wave to bend.
Diffraction - the spreading out of waves through an opening or around the edge of an obstacle (occurs with all types of waves)
Diffraction
Sometimes, waves interact with a partial barrier, such as a wall with an opening. As the waves pass through the opening, they spread out, or diffract. Sound waves can diffract as they pass through an open doorway. This can also occur as waves pass the edge of an obstacle. For example, water waves sometimes diffract when they pass large rocks in the water. Light waves diffract around the edge of an obstacle too. The edges of a shadow appear fuzzy because of that diffraction The light waves spread out, or diffract around the object that is making the shadow.
Waves interact with other waves
Waves adding together
If two identical waves coming from opposite directions come together at one point, the waves' crests and troughs are aligned briefly and join up exactly. When they merge into a temporarily larger wave, their amplitudes are added together. When the waves separate again, they have their original amplitudes and continue in their original directions.
Interference - the meeting and combining of waves
constructive interference - the adding of two waves
destructive interference - when the meeting of two waves result in the canceling out of the amplitudes
Waves cancelling each other out
If two very similar waves come together, but the crest of one wave joins briefly with the trough of the other, the energy of one wave is subtracted from the energy of the other. The new wave is therefore smaller than the original wave. If the amplitudes of the two original waves are identical, the two waves can cancel out completely. When identical waves meet, they are usually not aligned. The crests meet up in some places and troughs in others. As a result, the waves add in some places and subtract in others.