Waves Flashcards
Which direction do the particles oscillate in a longitudinal wave?
In the same direction as the energy transferred, they oscillate around a fixed point
Which direction do the particles oscillate in a transverse wave?
At 90 degrees to the direction of energy transfer
What is the equation to work out time period?
1/f
What is 1 wave cycle?
360 degrees
What is phase difference?
the difference in the position or progress of two waves at a given time
What is an example of a longitudinal wave?
Sound
What are examples of a transverse wave?
The electromagnetic spectrum
water ripples
How do you work out the speed of a wave?
C=fλ
What is the speed of light?
3.00x10^8m/s
What waves can be polarised?
Transverse waves
What happens when light is polarised?
The electric field vibrates in only one direction (plane)
What is polarisation useful for?
Transmitting via radio waves e.g. antennas
Sunglasses
What is a node?
A position where there is no displacement
What is an antinode?
A point of maximum displacement
What is interference?
When one wave interferes with another
What is superposition?
When you add 2 displacements of a wave together to get the final displacement
What happens when you have a wave which is half a wave cycle out of phase with another one?
They cancel each other out
What happens when you have a stationary wave?
The wave travels in one direction, bounces off the end and then interferes with itself
The two waves which are combining are coherent
What is coherence?
When 2 waves have the same phase difference, similar amplitude and same wavelength and frequency
When do we get constructive interference?
When the path difference is 0 or a multiple of a wavelength nλ
When do you get maximum destructive interference?
Where waves are half a wavelength out of the other one (n+1/2)λ
How do you work out frequency in a string?
1/2lx√T/μ
T=tension
μ=mass per unit length
What is an experiment for measuring microwaves?
Have transmitter, opposite metallic plate so as wave comes out, reflects and goes back again
Put in microwave detector, as moves back and forth we can look at size of detected signal, 0 at end, maximum in middle where antinode is
How can we measure waves with a tube?
Speaker on one end, tube with dust and plunger at one end which can be moved back and forth
When length is just right, particles form piles
Sound wave moves along and reflects off the end and interferes with itself
Also use Ruben’s tube with fire leaking at the top
What are properties that waves have?
What happens to sound when the interference is constructive?
It becomes louder
What happens to sound when the interference is destructive?
Peaks and troughs cancel out so sound becomes quieter
How do you carry out Young’s double slit experiment?
Shine laser (monochromatic) light through double slit
Laser light was similar wavelength to gap, so maximum diffraction through gap
Shine onto screen and get diffraction patten of fringes of light(alternating maxima and minima)
Measure width of fringes W, distance from screen D, distance between slits S
W=λD/S
How do you get wider fringes?
Put screen further away
Use longer wavelength of light
Decrease slit separation
Why do we get light and dark fringes?
Points of constructive interference - light
Points of destructive interference - dark
What happens when you diffraction from a single slit?
Bright central maxima, dark point of destructive interference, duller and narrower bands either side
What happens when you shine white light through single slit?
Bright white central maxima
Destructive interference either side thendifferent wavelengths of light diffracted and spread out, rainbows
Shorter to longer wavelength
What happens when you shine laser light through a diffraction grating?
Central maxima
Large region of no constructive interference
Repeating bright spots then other regions
What equation links distance between slits in diffraction gratings, order, wavelength and angle?
dsinθ=nλ
When do you get maximum constructive interference?
When path difference = λ
What is refraction?
When a wave slows down or speeds up and changes direction when passing from 1 medium into another
What is Snell’s law?
n1Sinθ1=n2sinθ2
What is the equation to work out the refractive index of a material?
n=C/Cs
Speed of light in a vacuum/speed of light in that material
Equation when light goes from denser material to less dense material
n1sinθc=n2
Sinθc=n2/n1
To work out critical angle, look at ratios of refractive index of n2 and n1
When do you get total internal refraction (TIR)?
When the angle hitting the boundary (angle of incidence) is bigger than the critical angle, so no light escapes and instead is internally reflected
How do we make sure light is totally internally reflected in an optical fibre?
Add cladding on outside
Step index
What are problems with optical fibres and solutions?
Some of light is absorbed, signal at other end is a bit lower
Have booster station to boost signal back up to original value so data can be transmitted over long distances
Pulse broadening, signal at other end is shorter and wider due to modal and material dispersion
Use other optical fibres to focus as much of the beam down the centre as possible
What is material dispersion?
Some frequencies of light travel quicker than others so some parts arrive before others
What is modal dispersion?
Some of the signal may be travelling in straighter lines, signals arrive at different times as they take different paths
