4.4 Waves Flashcards
Define the term “progressive wave”
A wave which transfers energy from one place to another with a wave front which travels through a material (in contrast to stationary waves which don’t appear to move).
What is a longitudinal wave?
Waves which the particles oscillate in the same direction as the energy propagation
There are rare fractions (areas of low pressure) and compressions (areas of high pressure)
What is a transverse wave?
Waves where the particle oscillations are perpendicular to the energy propagation
For example: electromagnetic waves
Define frequency and give its units
The number of waves passing through a point per second
Units of Hertz (Hz) or s^-1
Define wavelength
The distance between two adjacent corresponding points (eg. peaks) on a wave
Define amplitude
The maximum displacement of the wave from its equilibrium position
How can you find out the time period of a wave using its frequency?
T = 1/f
What is the phase difference of a wave and what is it measured in?
The amount one wave lags behind another as a proportion of the wavelength. Measured in radians or degrees l
What do each of the “axes” on an oscilloscope measure?
Vertical divisions = voltage/amplitude of the wave
Horizontal divisions = time (can be used to find the time period and frequency)
Define refraction
Refraction is when a wave bends at a boundary between two materials due to the difference in density causing it to speed up or slow down
True or false: A wave can either be refracted or reflected at a boundary, but never both
False
At a low angle of incidence most will be refracted, but some will reflect
True or false, diffraction is most noticeable when the wavelength is much larger than the gap the wave is travelling through
False
The most diffraction is seen when the gap and the wavelength are the same size
If the wavelength is much bigger the waves will be mostly reflected
Can all waves be polarised?
No. Only transverse waves
What is the difference between a polarised and unpolarised wave?
Polarised waves only contains waves oscillating along one axis
Unpolarised waves can be oscillating in any direction perpendicular to the axis of propagation
Describe how a ripple tank might be used to investigate diffraction
Create water waves in the tank
Vary the size of the gap for them to pass through
Note how the direction changes of the waves passing through
Polarised light is being passed through a rotating polarisation filter. What would happen to the intensity of the light passing through?
It would vary from a maximum (all light passes through) when the axis of polarisation and the axis of the filter line up to a minimum (no light passes through) when the axes are perpendicular
How is intensity defined in terms of power?
Intensity is power / area
How are intensity and amplitude related?
Intensity is proportional to amplitude^2
True or false: All electromagnetic waves have the same time period
False
They all travel at the same speed - but their wavelengths, frequencies and time periods vary
How fast do electromagnetic waves travel in a vacuum?
3 x 10^8 m/s (“the speed of light”)
True or False? The magnetic field and electric field in an electromagnetic wave are parallel to each other
False
The electric and magnetic field are at right angles to each other
Put the following in order from highest to lowest frequency:
X-rays, Radio, Microwaves, UV, Visible
X rays, UV, Visible, Microwaves, Radio
Which type of EM radiation has a wavelength of approximately 1micrometre?
Infrared is between ~700nm and 1micrometre
What is the range of wavelengths commonly known as “visible light”?
300-700nm
True or False: Microwaves can be polarised using a metal grid rather than a polarising filter
True
This is because the wavelength of the microwaves is sufficiently larger than the polarising filter slits, therefore a grid would work instead as it’s difference is similar to the wavelengths of microwaves
What is meant by the refractive index of a material? What equation can be used to find it?
The refractive index is a measure of how fast light travels in a material compared to its spread in a vacuum
It is found using: n = c/v
n = refractive index c = speed of light v = velocity in the material
A beam of light is shone at a boundary between air and glass. As the angle of incidence is increased from 0 to 90, what would you see?
At 0° all of the light would pass into the material along the normal
The light would then be seen as to be refracted (the angle to the normal in the glass would be larger than the incident angle)
Eventually the light would bend so much that it would start to be reflected back
What is the name given to the angle of incidence at which light will reflect off a boundary rather than refracting in the medium?
The critical angle
sinC = 1/n
C = critical angle n = refractive index of the material being passed into
What is the name given of what happens to light at angles greater than the critical angle?
Total internal reflection
Waves can “superpose” - what does this mean?
Two waves in the same place (ie. one on top of the other) will combine
What are the two types of interference?
Constructive and destructive
Describe an experiment to investigate the principle of superposition using sound
1) Use two speakers, a moderate distance apart, connected to the same signal generated to transmit sound waves
2) Walk along a line perpendicular to the speakers - you should hear alternating loud and quiet points
3) This is because in some places the waves from each speaker constructively interfere (loud) and some places it’s destructive (quiet)
Define coherence
Coherent waves have the same frequency and wavelength and a fixed phase difference (often zero in exam questions)
Of two waves are in phase will they constructively or destructively interfere?
Constructively
True or false: Path difference and phase difference are two names for the same thing
False
Path difference is the difference in distance that two waves have travelled in terms of the wavelength (units of length)
Phase difference is the difference in the point in the cycle of two waves as a proportion of a full wave cycle (units of degrees/radians)
Why is a layer useful in showing interference and diffraction?
It produces monochromatic (same wavelength/colour) light
What is Young’s double-slit experiment?
A single source of light directed towards a double slit, which creates two coherent beams of light. argus interferes as it hits the screen and creates an interference pattern
Describe the interference pattern created using white light
The interference pattern would be a repeating coloured spectrum along the screen, with a bright white point directly in front of the slit
True or false? Increasing the slit width increases the width of the central diffraction maximum
True
What equation relates to the wavelength of light to the slit spacing and distance to the screen
λ = ax/D
λ = wavelength a = slit spacing x = fringe spacing (on screen) D = distance to screen
Which two properties of light can only be explained if it is a wave?
Diffraction
Interference (as seen in Young’s experiments)
When shining light through a diffraction grating there is a maximum number of fringes that would be produced. How would you find this maximum number?
nλ = DsinØ
The maximum angle that would produce a fringe would be 89.999999999… (so call it 90!)
Rearrange the equation for n, using Ø=90
What is a stationary wave?
Stationary waves consist of alternating fixed pattern of nodes (points with zero amplitude) and antinodes (points with maximum amplitude). No energy is transferred across the wave
What is a node?
A point with no vibrations in it which the resultant amplitude is 0
What is an antinode?
A point with maximum vibration in which the resultant amplitude is at maximum
What are the conditions for a stationary wave to be produced?
The waves must be coherent
They must be travelling in opposite directions
These conditions are often met when a wave is reflected back onto itself
Give an example of an experiment you would do to show a stationary wave
Use an oscillator to pass a wave along a string which is fixed at one end
The stationary wave will form when the progressive wave is reflected off the fixed end
Give a similarity and a difference between stationary waves and progressive waves
Similarity: Both have wavelength, frequency, amplitude
Difference: Stationary waves don’t transmit energy from one place to another
How could you use formation of stationary waves in a resonance tube to find the speed of sound?
1) Create a closed end pipe using a hollow pipe inside a measuring cylinder containing water
2) Use a tuning fork (producing known frequency) and hold it above the tube
3) Move the tube up until you find the first position which causes resonance
4) This length will be a quarter of the wavelength
5) Use speed = frequency x wavelength
What is meant by harmonics?
Harmonics are points where the stationary wave form doesn’t change because the waves in each direction are reinforcing each other
A stationary wave on a string is made to oscillate at its fundamental frequency (1st harmonic) - how many nodes and antinodes would you see?
Nodes - 2 (1 at each end)
Antinodes - 1 (in the middle)
