11 - Waves 1 Flashcards
Define progressive wave.
An oscillation that transfers energy but doesn’t transfer matter.
Define mechanical wave.
Progressive waves that require a medium to travel through.
Define electromagnetic wave.
Progressive wave that can travel through a vacuum.
How do mechanical waves move?
.Movement of matter transfers energy.
.Particles move from equilibrium position to new position as the wave travels through the medium.
.This requires a force which they exert on their neighbours like springs.
Define transverse wave.
Energy transported perpendicular to the direction of oscillation.
Define longitudinal wave.
Energy transported parallel to the direction of oscillation.
What are the movements of a longitudinal wave called?
Compression and rarefaction.
What are some examples of longitudinal waves?
.Sound waves.
.P-waves.
.Pressure waves.
What are some examples of transverse waves?
.EM waves.
.Water waves.
Define amplitude.
The maximum displacement from the equilibrium position (m).
Define wavelength.
The maximum distance between two points in phase on adjacent waves (m).
Define frequency.
The number of waves passing a point per unit time (Hz/s^-1).
Define time period.
The time taken for a wave to move past a given point/time taken for one oscillation (s).
How are time period and frequency related?
Inversely proportional - f = 1/T
Define displacement.
Distance travelled in a specific direction.
What is the wave speed equation?
v = fλ.
What speed do EM waves through a vacuum travel at?
3 x 10^8 ms^-1.
What is another equation for wave speed?
v = λ/T.
Define phase difference.
The difference between different displacements along a wave/difference in displacements of different waves.
How do you measure phase difference?
As they are oscillations - in angles in radians.
Define in phase.
When a part continually has the same displacement - either 0 or 2pi radians.
Define antiphase.
When a part continually has the opposite displacement.
How do you work out phase difference in degrees?
= x/λ x 360.
Define ray.
Representation of a wave - straight line with an arrow indicating energy transport direction.
Define reflection.
When a wave changes direction at a boundary between two materials.
Define refraction.
When a wave changes direction and speed when passing from one material to another.
What are the laws of reflection?
.Angle of incidence = angle of reflection.
.Wavelength and frequency don’t change.
.Reflected wave gains additional 90 degrees phase difference from the incident wave.
What happens in refraction?
.Changes velocity.
.If bends towards normal, slows down.
.If bends away, speed increases.
.When wave slows down - wavelength decreases but frequency is constant.
Why do waves slow when wavelength decreases?
.E = hv/λ.
.λ = hv/e
.So they are directly proportional when E is constant.
What acronym helps with refraction?
Fast.
Away.
Slow.
Towards.
What is diffraction?
.When waves spread out after passing through a gap.
.Done by all waves.
.Speed, wavelength and frequency don’t change.
.Only direction changes.
What does the magnitude of diffraction depend on?
.Wave wavelength.
.Gap size.
The more comparable they are, the larger the magnitude of diffraction e.g. light/sound with a door.
Define plane polarised waves.
Waves which oscillate in one plane.
Define unpolarised waves.
waves oscillating in many possible planes e.g. filament lamp light.
Explain why longitudinal waves can’t be plane polarised.
.Energy transport is parallel to oscillations.
.All oscillations are limited to one plane.
.Therefore, can’t be plane polarised.
What happens to reflected transverse waves?
.Become partially polarised - more waves oscillating in one plane but not fully.
.Sunglasses - block out reflected partially polarised light but not unpolarised light.
How do polarisers work with EM waves?
.Vertical polarisers allow vertically aligned fields pass through. Vice versa with horizontal polarisers.
MICROWAVES ARE THE EXCEPTION AND BEHAVE IN THE OPPOSITE WAY DUE TO ELECTRON MOVEMENT.
What are some uses of polarisers.
.Communication polarisers - reduce interference.
.3D cinema glasses.
What experiment shows polarisation?
.Take 2 filters and light and the light measuring thingy.
.Rotate 1 filter.
.Measure again and repeat.
.Should produce cosine wave looking pattern but above x-axis.
Define intensity.
The radiant power through a surface with unit area.
What are the intensity equations and units?
I = P/A
I = P/4pir^2
I = Wm^-2.
What is the relationship between intensity and amplitude?
Intensity is proportional to Amplitude^2.
What is refractive index?
.n = c/v
.No units.
What is the refractive index in air?
n = 1.0003.
Why is n always greater than or equal to one.
The velocity must always be less than the speed of light as waves can’t travel faster than c in a vacuum.
What are the conditions for Total Internal Reflection?
.Light must be travelling through a medium with a higher n and hits a boundary of a material with a lower n.
.The angle at which the light strikes the boundary must be above the critical angle which depends on the material’s n.
What happens when the angle is below/equal to/above the critical angle?
.Some reflected/refracted.
.Sent along the boundary.
.TIR.
What are the equations for TIR?
sin0c = 1/n1 or n1 = 1/sin0c.
How does fibre optics work?
.Light travels down due to TIR.
.Used in communication and medical endoscopes.
How does cladding work?
.Reduces light loss by giving a 2nd chance at TIR.
.Increases critical angle but faster if only travels through the core.
.If it has a lower n than the core, TIR occurs.
.If it has a higher n, refraction occurs.
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