Waves Flashcards
Progressive Wave
A wave which travels continuously in a medium in the same direction without a change in amplitude.
What are waves? What do they do?
Oscillations of particles or a field.
They transfer or store energy.
What does a progressive wave do?
Transfers energy
2 Types of progressive wave?
Longitudinal and transverse
Longitudinal wave
Particles oscillate in the same direction as the energy transfer. The particles do not move but oscillate around a fixed point.
Which way is energy transferred in a longitudinal wave?
—-> The same direction the particles vibrate.
How do particles oscillate in transverse wave?
Particles oscillate at 90 degrees to the direction of energy transfer (—–>).
A wave with a longer time period has a lower…
frequency
Examples of longitudinal waves
Sound waves and ultrasound
Examples of transverse waves
Electromagnetic spectrum, waves on a string (can be polarised), water ripples(can be polarised).
In an EM spectrum the waves travel at…
the speed of light.
(polarisation) To stop a transverse wave getting through you need…
two filters at 90 degrees.
Can you polarise a longitudinal wave, why?
No because it oscillates back and forth so it can fit through the filters.
What can polarise light?
things like sunglasses
What is polarising light useful for?
Transmitting things with radio waves e.g a house with horizontal ariels so the waves for things like tv will be horizontally polarised. Vertical ariels only pick up vertically polarised waves.
What is a stationary wave?
Combination of two waves moving in opposite, each having the same amplitude and frequency.
What is a node?
A position where there is no displacement.
Antinode is the point of…
maximum displacement
First Harmonic
A single loop, made up of two nodes and an antinode.
What do harmonics depend on?
The frequency of vibration and the situation they are created.
(Harmonics) As the frequency is increased, what happens?
More harmonics appear.
Distance from node to node is equal to…
wavelength / 2
Interference
When one wave interferes with another.
Superposition
Two or more of the same wave type crossing each other to get a final displacement by adding each displacement.
What type of interference is at each node?
Antinode has constructive interference
Node has destructive interference.
Constructive interference
When maxima of two waves in phase add together, the amplitude of resulting wave is equal to the sum of individual amplitudes.
Destructive interference
Where two waves are out of phase and cancel each other out.
There is always constructive interference when the…
path difference (m) is equal to 0 or a multiple of a wavelength.
When there is CI, the path difference is equal to…
n (number) x wavelength.
Unit for path difference
Metres
Unit for phase difference
Degrees or radians
Where there is DI, something is equal to…
(n+1/2) x wavelength
Equation for frequency of First Harmonic
f = (1/2L) x Square root of T/μ
L = distance from one end to the other.
2L = Wavelength of the wave.
T = Tension in string
μ = Mass per unit length (m/L)
Higher tension in a string means…
higher frequency
Example of a standing wave
Rubens Tube, demonstrates acoustic standing waves in a tube.
What is Diffraction?
The spreading out of waves as they pass through an aperture or around objects
When does Diffraction occur?
It occurs when the size of the aperture or obstacle is of the same order of magnitude as the wavelength of the incident wave.
What is a coherent wave?
Coherent waves have the same frequency and a constant phase difference.
What type of light is Laser?
Monochromatic
Monochromatic light
All has the same wavelength
When is there maximum diffraction through a gap?
When the gap is approximately a similar size to the wavelength of the wave going through it.
Width of fringe?
Distance between two consecutive bright or dark fringes. (w)
(D)istance (double slit)
Distance from double split to the screen. (D)
Distance (s)
Distance between slits. (s)
How do you get wider fringes?
Put the screen further away.
use a longer wavelength of light.
Decrease slit separation.
What happens if you shine white light through a single slit?
A bright white central maxima, then a dark fringe and the spectrum of light continues each side.
Refraction?
Refraction is caused by a wave slowing down or speeding up. It changes direction as it passes from one medium into another.
Path difference
The difference in distance travelled by two waves from their sources to the point where they meet
What is pulse broadening?
When the duration of a pulse increases because of dispersion. Each pulse lasts a longer time when it is received than when it was sent
The consequence of pulse broadening?
Overlapping pulses cannot be easily distinguished, limiting the maximum frequency and so limiting the data that can be sent
What is absorption?
When the energy of a signal is absorbed by the optical fibre in which it travels
The consequence of absorption
The further the light travels, the weaker the signal becomes
What is material dispersion?
The spreading of a signal caused by the variation of refractive index with wavelength
What is modal dispersion?
The spreading of a signal caused by rays taking slightly different paths in the fibre
What is fibre optic?
A very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners using TIR
What is the function of cladding?
To keep the signal secure, to reduce pulse broadening which reduces signal degradation, to keep most light in the core due to TIR at the cladding-core boundary
What is total internal reflection?
When the angle of incidence is greater than the critical angle refraction can’t happen therefore, all light is reflected back into the material
What is the refractive index of air?
1
Define the principle of superposition
When 2 or more waves pass through each other, the displacements due to each wave combine
Explain the function of polaroid material
If you view partially-reflected light through a polariser at the right angle, you can block out some of the reflected light. This reduces the intensity of light in your eye
Explain polarisation as evidence for the nature of transverse waves
Polarisation can only happen for transverse waves as the direction of propagation and the direction of vibration must be different
Why are optical fibres narrow?
to reduce modal dispersion
How do we reduce material dispersion?
by using monochromatic light
What does partially polarised mean?
some of the light vibrates along one direction
the relationship between refractive index and optical density
the higher a mediums refractive index the higher its optical density
Feature of cladding
lower refractive index to allow TIR
What happens when light diffracts through a single slit?
causes the wave to interfere with itself
In single slit diffraction what causes the interference?
the path difference between the top of the slit and the bottom of the slit
How to find the highest order of maximum?
light cannot diffract by >90°
What is diffraction used for?
measure wavelength of light, chemical analysis, astrophysics, to find the structure of crystalline materials
How are diffraction gratings used in fibre optics?
monochromators use different diffraction gratings to produce monochromatic light to avoid material dispersion
