Waves Part 1 Flashcards
What is a progressive wave?
- A progressive wave transfers energy without transferring material.
- The wave is made up of particles of the medium that it is travelling through OR fields oscillating.
Example of how you known progressive waves transfer energy?
- EM waves are progressive waves and they cause objects to heat up
- X-rays + Gamma rays cause ionisation
What is the ‘displacement’ of a vibrating particle of a wave?
- The distance and direction (+/-) of the wave from the equilibrium (undisturbed) position.
- Units: m
What is the ‘amplitude’ of a wave?
*Look at diagram in notes
- The maximum displacement of a wave from the equilibrium position.
- Units: m
What is the ‘wavelength’ of a wave?
*Look at diagram in notes
- The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (e.g. the distance between successive peaks)
- Units: m
What is the period of a wave?
The time taken for one complete wave to pass a fixed point.
Units: s
What is the ‘frequency’ of a wave?
- The number of complete oscillations passing through a fixed point each second
- Units: Hz
What are transverse waves?
Waves in which the direction of oscillations (particles or fields) are perpendicular to the direction of energy transfer.
Examples of transverse waves?
- EM waves
- Secondary seismic waves
- Waves on a string/rope/slinky
- Water ripples
What are EM waves?
EM waves are waves which consist of electric and magnetic fields oscillating in phase and at right angles to each other and to the direction of energy transfer.
What are longitudinal waves?
Waves in which the direction of oscillations (particles) are parallel to the direction of energy transfer.
What do longitudinal waves consist of?
- Longitudinal waves consist of compressions and rarefactions.
- Look at notes to see compressions and rarefactions*
Can longitudinal waves travel in a vacuum?
Longitudinal waves require a medium in order to travel so unlike transverse which can travel in a vacuum, longitudinal waves cannot.
Examples of longitudinal waves?
- Sound waves
- Primary seismic waves
What is unpolarised light?
Unpolarised light is light which consists of waves that are oscillating in more than one plane.
Polarised light is light which consists of waves that are oscillating in only one plane.
Unpolarised light can be polarised. What type of waves only can be polarised?
Only transverse waves can be polarised. (Longitudinal waves cannot be polarised).
The fact that polarisation is possible is evidence for the nature of transverse waves.
What is needed to polarise light waves and what does this do?
Unpolarised light can be polarised using a polaroid filter. When unpolarised light passes through this filter, the transmitted light is polarised as the filter only allows light which vibrates in a single direction/plane (according to the alignment of the molecules in the filter) to pass through. The transverse waves are now plane-polarised.
What happens when light passes through two polaroid filters at right angles to each other?
No light would be transmitted at the other end.
When does light become partially polarised?
- Light is partially polarised when it is reflected from some surfaces.
- This is such that waves parallel to the plane of incidence are transmitted (at a greater intensity) after reflecting, in comparison to waves in other planes.
*IS THIS CORRECT? If so why? Explain this further.
What does partially polarised (after reflection) mean?
Light is said to be partially polarized after reflection when one of the planes of the incoming wave is expressed at a greater intensity than the others following reflection. Reflection can cause partial polarisation as the waves which are parallel to the plane-of-incidence are expressed at a higher intensity, compared to the waves in the other planes - other planes do not cancel out, only reflected at a lower intensity.
Fully polarised = other planes completely cancel out.
How do polaroid sunglasses make use of the fact that reflected light are partially polarised.
- Glare is when light reflects off surfaces into our eyes, blocking our vision.
- The fact that it gets partially polarised reduces the impact of this glare (reduces intensity of light hitting eyes).
- However to completely eliminate the glare, polaroid glasses make use of a polaroid filter with a vertical orientation to allow through the vertically polarised light only (while blocking out any waves in the horizontal plane which is the majority of the partially plane polarised light following refelection).
What applications do polarisers have?
1) Polaroid material
2) Alignment of aerials for transmission and reception.
Why are polarisers important in the alignment of aerials for transmission and reception.
TV and radio signals are usually plane-polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation as the rods on the transmitting aerial to receive the signal at full strength.
Define wave speed?
Distance travelled by the wave per unit time.
Units: m/s
For a constant speed, the higher the _______ of a wave, the lower the _______ of the wave.
1) Frequency
2) Wavelength
*Look at EM waves for example, same speed, different frequencies and wavelengths.
How to calculate period of a wave?
*Period = time taken for 1 complete wave to pass a fixed point.
Period = 1/f
…where f is the frequency of the wave in Hz
How to calculate the frequency of the wave given the period?
Frequency = 1/period
How to calculate the speed of a wave?
c = fλ
speed of wave(m/s) = frequency(Hz) x wavelength(m)
*c is not the speed of light unless an EM wave, just the wave speed.
What is the phase of a vibrating particle?
The phase of a vibrating particle at a certain time is the fraction of the cycle it has completed since the start of the cycle.
Units: radians, degrees or fractions of a cycle/wavelength
What is phase difference?
How much a particle/wave lags behind another particle/wave.
Units: radians, degrees or fractions of a cycle/wavelegnth
*If comparing between two waves, these two waves are usually travelling at the same speed.
What is the phase difference between two vibrating particles of the same wave that are a wavelength apart?
*Also used to compare between two different waves.
The phase difference is 360°/0° or 2π/0π radians
What is the phase difference between two vibrating particles of the same wave that are half a wavelength apart?
*Also used to compare between two different waves.
The phase difference is 180° or π radians
What is the phase difference between two vibrating particles of the same wave that are quarter a wavelength apart?
*Also used to compare between two different waves.
The phase difference is 90° or 1/2π radians
For two seperate waves, what phase difference do they need to be IN PHASE?
0°, 360° or any multiple of 360°
For two seperate waves, what phase difference do they need to be exactly OUT OF PHASE?
180° or any phase differences that are equal to an odd number x 180° (180° x 3, 180° x 5, etc)
What do you call waves that are exactly out of phase (out of phase by 180° etc)
The two waves are in antiphase.
What would these degrees be in ‘fractions of wavelength’?
90° 180° 270° 360°
90° = 1/4λ 180° = 1/2λ 270° = 3/4λ 360° = 1λ
What is the phase difference between two vibrating particles of the same wave that are three quarters a wavelength apart?
The phase difference is 270° or 3/2π radians.
How to convert fractions of wavelength into degrees or radians?
Into degrees: fraction of wavelength x 360
Into radians: fraction of wavelength x 2π
How to convert degrees to radians and vice versa
degrees x π/180 = radians
radians / π/180 = degrees
What can all waves do?
Waves can refract, reflect and diffract
To observe the properties of waves, we can use a ripple tank. What are ‘wavefronts’ on ripple tanks?
These are lines of constant phase (they highlight the corresponding parts of a wave that are in phase).