Physics Waves Flashcards
What is a cycle?
One complete vibration of a wave
What is displacement in the context of waves?
How far a point on the wave has moved from its undisturbed position.
What is amplitude?
Maximum magnitude of displacement
What is wavelength?
The length of one whole wave cycle from two similar points.
What is a period?
The time taken for a whole cycle to complete or to pass a given point.
What is frequency?
The number of cycles per second passing a given point
What is phase?
A measurement of the position of a certain point along the wave cycle
What is phase difference?
Amount that one waves lags behind another- measured in angles or radians (Pi)
What is reflection?
When a wave is bounced when it hits a boundary
What is refraction?
When a wave changes direction as it enters a different medium, resulting in the wave speeding up or slowing down
What is a longitudinal wave?
They are waves that vibrate along the direction energy transfer, they consist of compression’s are rarefaction’s
What is a transverse wave?
Transverse waves are waves that vibrate at right angle to the direction of energy transfer, all electromagnetic waves are transverse.
What is the proof that electromagnetic waves are transverse?
A polarised wave only vibrates in one direction, only transverse waves behave like this so proof that they are transverse
What do polarisation filters do?
They only transmit vibrations in one direction
How are polarising filters used in sunglasses?
The polarising filter is at right angle to the direction of the vibration, this results in glare being reduced as when the light becomes polarised, and reflects onto the glasses only the light that has the same vibration interference as filter will be affect the glasses. This results in blocking unwanted glare as it’s at a certain angle.
Give an example where polarisation is used apart from sunglasses
Radio signals and Television
What is constructive interference?
When two waves are in phase and have the same frequency, resulting in the forming into one with the combined amplitudes.
What is destructive interference
When two waves are out of phase and have the frequency, resulting in the cancellation of the wave, due to the negative and positive displacements canceling out.
What is superposition?
Superposition is when two or more waves interfere, the resultant displacement equals the sums of the individual displacements.
For interference to be noticeable what must be?
The two waves must have a similar amplitude.
What does in phase mean?
If two waves are at the same point in the wave cycle
Points in phase have the same what?
Displacement and velocity
To get interference patterns the sources of light must be ?
Coherent
What is coherent light?
If they have the same wavelength, frequency and fixed phase difference.
What happens when there is a path difference of half a wavelength, one and a half wavelength, two and a half ?
Destructive interference occurs as they are out of phase
How are stationary waves produced?
A progressive wave is reflected at a boundary, when the wave is reflected it interferes on the reflect with another progressive wave if they have the same frequency due to the theory of superposition. As they are in phase the progressive waves constructively interfere resulting in the formation of a point of maximum amplitude called an anti-node, continually as the whole wave is not in phase points are out of phase on the progressive wave this results in destructive interference also occurring, when they interfere it creates a point of minimum displacement as the positive displacement and the negative displacement interfere resulting in a node being created.
No energy is transmitted by what type of wave?
Stationary waves
What is the first harmonic?
When the stationary wave is vibrating at the lowest possible resonant frequency. It has one “loop” with a node at each end.
What is the second harmonic?
It’s twice the frequency of the first harmonic, there are two loops with a node in the middle and one at each end. This is one whole wavelength
What is the third harmonic?
The third harmonic is three times the frequency of the first harmonic. 1 and 1/2 wavelength
How can powder show stationary waves?
Lycopodium powder laid along the bottom and the power is shaken away from the anti-nodes but left undisturbed at the nodes.
The amount of diffraction depends on what?
The size of the gap in comparison to the wavelength
When does the most diffraction occur?
When the gap is the same size as the wavelength
What happens when a monochromatic coherent light is shone through a narrow slit?
When a monochromatic coherent light is shone through a narrow slit this can form diffraction patterns, central bright fringes, with dark and bright fringes alternating on either side. The dark and bright fringes are caused by destructive and constructive interference of light waves.
What happens when white light is shone through a slit?
It creates a central maxima point that is white that is surrounded by a spectra of colours, this also results in the black fringes being closer together.
If something has a high intensity it has a high amount of ?
No. of photons
Why is the central maxima the brightest?
It has the highest intensity as it’s where the light constructively interferes.
In monochromatic light photons have the same ?
Energy
An increase in intensity means an increase in what? (For monochromatic light)
An increase on no. photons per second.
If you increase the slit width what happens to the amount of diffraction?
It decreases the amount of diffraction meaning the central maximum is more narrow and this results in an increase in intensity
If you increase the wavelength what happens to the amount of diffraction?
It results in an increase in the amount of diffraction meaning the central maxima is wider resulting in the intensity of the central maxima being lower.
For two source interference in water and sound what do you need?
Need coherent sources
Need to use the same oscillator
What is coherent?
Same wavelength and frequency
In Young’s Double Slit experiment what is the danger with working with lasers?
Damage to the retina-(Wear laser safety goggles), Reflections could harm people outside without laser safety goggles
What is the fringe spacing formula?
w=Wavelength*D/s D= distance from slit to screen s= Slit spacing
What is use of a diffraction grating?
To help identify elements and calculate atomic spacing
What happens if you diffract a white light?
White light is a mixture of colours, if you diffract white light you get a spectra due to all the different wavelengths being spread out different amounts.
In each spectrum red is on the outside and violet on the inside, the central maxima stays white.
What is the refractive index?
A measure of how much light slows down
Where is light fastest, why does it slow down?
Light is the fastest in a vacuum, it slows down in other materials due to interacting with particles in them.
What is the refractive index in air?
1
What is the angle of incidence in the context of refractive index?
The incoming light, before its refracted
What happens if you increase the angle of incidence?
You increase the angle of refraction what limits out at 90*
What happens when the angle of incidence reaches the critical angle?
If it reaches above the critical angle refraction is impossible all light is reflected back resulting in total internal reflection.
What is the use of cladding in optical fibers?
Cladding has a low refractive index allows internal reflection to occur, it also protects the fibre from getting damaged.
Explain how light travels in optical fibres?
Light is shone in at one end of the fibres, ensuring its narrow so that light always hits a boundary between the fibre and cladding at an angle bigger than the critical angle so that all light totally internally refracts at each boundary.
What is modal dispersion?
Light rays enter fibre at different angles causing different paths.
What is a single mode fibre and what is its use?
A single mode fibre only allows light to take one path to stop modal dispersion.
What is a material dispersion?
Light consists of different wavelengths that travel at different speeds this results in some light reaching the end of the fibres faster than others.
How can you stop material dispersion?
Monochromatic light as it will have the same frequency
What do modal dispersion and material dispersion both lead to?
Pulse broadening due to overlap confusing the signal.
