Chapter 4 - Waves Flashcards
What’s is mean position of a wave?
It’s the fixed average point of the particles of the wave
Amplitude
The maximum displacement from the mean position
Frequency
Number of oscillations completed in one second
Unit: Hz
Time period
Time taken for one complete oscillation
Phase difference
Measures in degrees or radians, how ‘in step’ the different molecules are
In phase: moving together
Out of phase: moving separately
What’s one complete cycle and half a cycle?
360 degrees/2 Pi = one complete cycle
Pi = half a cycle
90 degrees/ Pi/2 = quarter of a cycle
Continuous wave
Involves a succession of individual oscillations
A wave pulse
Involves just one oscillation
Longitudinal waves and examples
Oscillations are parallel to the direction of energy transfer.
E.g sound waves and earthquake waves
Transverse waves and examples
Oscillations are perpendicular to the direction of energy transfer
E.g light waves, water ripples and earthquake waves.
Transverse mechanical waves cannot be propagated through fluids.
Wave equations
Wave speed (velocity) = frequency x wavelength
Displacement
Measures the change that has taken place as a result of a wave passing a particular point.
Zero displacement refers to the mean position
Wave speed
The speed at which the wave fronts pass a stationary observer.
Symbol: c
Unit: ms^-1
Intensity
Power per unit area that is received by the observer
Unit p: Wm^-2
Intensity is proportional to the square of its amplitude
What is the electromagnetic spectrum and its properties?
Gamma rays, x-rays, UV, visible, IR, microwaves and radio waves.
All transverse waves, can travel through a vacuum. All have the same speed 3.0 x 10^8 ms^-1
How are magnetic fields involved with the electromagnetic spectrum?
Accelerating charges generate electromagnetic fields. If a a electric charge oscillates it will produce a varying electric and magnetic field perpendicular to one another.
What is interference?
When two of the same type of wave meet. It can take place if there are 2 possible routes for a ray to travel from source to observer.
A resultant wave is the 2 waves added together
After the 2 waves meet they travel unaffected by their meeting.
When can there be areas of constructive or destructive?
If the waves have the same amplitude and frequency then the interference at a particular point can be constructive/ destructive.
What is constructive interference?
When the two waves are in phase and are always adding up. There is zero phase difference.
Path difference = n wavelength
What is destructive interference?
Takes place when the waves are out of phase. E.g the phase difference is half a cycle, or 180 degrees, or Pi radians
Path difference = (n + 1/2) wavelength
What is a phase change?
It’s when a trough comes back as a peak (the wave is flipped)
What’s a path difference?
The extra physical distance one wave might of gone that the other one didn’t.
What’s coherent?
The 2 waves have a constant phase relationship (but don’t have to be in phase) as the sources are constantly producing them. The frequency must be the same and stay that way.
What’s non-coherent?
Phase change occurs because waves aren’t continuously being produced.
The plane of vibration of an electromagnetic wave
The plane that contains the electric field and the direction of the propagation.
Polarised light
Over a period of time, the electric field only oscillates in one direction.
Unpolarised light
Over a period time, the electric field oscillates in random directions.
What is partially plane-polarised and circularly polarised?
1) . A mixture of polarised light and unpolarised light.
2) . If the plane of polarisation rotates uniformly
What is a polariser?
Any device that produces plane-polarised light from an unpolarised beam.
What is an analyser?
It’s a polarised used to detect polarised light.
What is a Polaroid?
A material which absorbs any light in one particular plane of polarisation allowing transmissions only in the plane of 90 degrees to this.