5.2 - The Behaviour Of Waves Flashcards
What’s the wave phase
All points on a wave will be at some position through their cycle of oscillation, some positions have their own names eg peak, crest, rarefaction or compression
We describe any position through a cycle by a number, referred to as the phase
What is a complete cycle or a wave equal to
It’s considered equivalent to rotation through a complete circle - so the phase position will be an angle measurement, where a complete cycle is equal to 360 degrees
What’s the alternative angle units to degrees
Radians
What’s one cycle equal to in raisins
One cycle is 2 pi radians
Radians can be shortened to rad
What are wavefronts
Diagrams of waves are often drawn as lines, where all the points in a line represent points on the wave that are exactly the same phase position, perhaps a wave crest for example
These lines on a diagram are call wavefronts
What is a ray on a wave diagram
The line showing the direction of travel is called a ray, rays must be perpendicular to wavefronts
What happens with wave superposition
When waves meet, each wave will be trying to cause a displacement at that point of intersection, according to its phase at that location. The net effect is that the overall displacement will be the vector sum of the displacements caused by the individual waves
After the encounter, each wave will continue past eachother, as the energy progresses in the same direction it was originally travelling - this can be simply illustrated if we consider just wave pulses passing each other - waves go back to original displacement after the intersection
What happens with the superposition of continuous waves
If rather than a single point along the path of the waves, we consider waves superposing over an extended space, the outcome is a continuous wave that is the sum of the displacements over time in each location
What’s constructive interference
If the two continuous waves are in phase, their effect will be to produce a larger amplitude resultant wave
What’s destructive interference
If identical waves meet and are exaclty out of phase, phase difference is 180 degrees, then the resultant is a zero amplitude wave
Define wavefronts
Wavefronts are lines connecting points on the wave that are at exaclty the same phase position
Define constructive interference
It’s the superposition effect of two waves that are in phase, producing a larger amplitude resultant wave
Define destructive interference
It’s the superposition effect of two waves that are out of phase, producing a smaller amplitude resultant wave
How can a stationary or standing wave be set up
Continuous waves travelling in the opposite directions will superpose continuously, this can set up a standing wave.
The waves need to be if the same speed and frequency with similar amplitudes and have a constant phase relationship
What’s coherent
Waves with the same frequency and a constant phase relationship are said to be coherent
Thus waves must be coherent in order for a standing wave to occur
What do stationary waves do
They are so called stationary waves as the profile of the wave does not move along, it only oscillates. This also means that wave energy doesn’t pass along a standing wave - they kind of don’t meet definition of waves, which do transfer energy and are more precisely called progressive waves
What are progressive waves
Waves that transfer energy
What are nodes and antibodies
On a standing wave,
Nodes are the points where the resultant displacement is always zero, thus points never move
Antibodies are points of maximum amplitudes
All points between one node and the next are In the same phase at all times although their amplitude of vibration varies up to the antipode and back to zero at the next node
This can be demonstrated in school, with a vibration generator and a rubber cord
At the end points on a stationary wave what happen
If the string / wave thing is fixed at each end, these points must always be nodes!!!
A standing wave can only occur if its wavelength exaclty allows one node at each end