Unit 3 - Waves Flashcards
What is a progressive wave?
A wave that carries energy from one place to another
What are the two types of progressive waves?
Longitudinal and transverse
What is the difference between longitudinal and transverse waves?
Longitudinal waves oscillate perpendicular to the propagation where as transverse waves oscillate perpendicular to the propagation
Define amplitude
The maximum extent of oscillation from the equilibrium position
Define wavelength
The distance between to equivalent points on a wave, measured in m
Define frequency
The number of waves that pass a given point in a specified time period, measured in Hz
T/F: particles move along a wave
False: particles oscillate either perpendicular or parallel to the direction of travel but do not move with the wave
Define a wave cycle
A portion of a wave between two wave crests or between two wave troughs or any other two identical points
How many radians are there in 180 degrees?
Pi
How do you convert degrees to radians?
Radians = (Degrees/180) x Pi
What is the phase difference between two waves that are in antiphase?
180 degrees
What is the phase difference between two waves that are in phase?
0 degrees
Give an example of longitudinal waves
Sound waves and seismic-P waves
Give an example of transverse waves
Water waves, seismic-S waves and EM waves
What are the two main parts of a longitudinal wave?
Compressions and rarefractions
What does it mean if an EM wave is plane-polarised?
Different components of a wave are oscillating in one plane only each
What does a polariser do?
It only lets through a certain component
T/F: A randomly orientated wave has a lower intensity then a plane polarised wave
False: A plane polarised wave has a lower intensity because not all components of the wave are still there
By how much does the intensity of a light wave decrease when it is polarised?
1/2
Give examples of where polarisers are used and why they are used
- Sunglasses: reduces the intensity of the light passing through them
- Technology: light emitted from screens is polarised because it makes it easier to see especially in bright sunlight
- 3D movies: a polariser is used in each side of the sunglasses, which corresponds to a specific wave emitted by the projector which means that each eye only sees one wave and you get a 3D effect
Define superposition
When two waves overlap and are added together to form one wave
Define destructive interference
When two points on two waves with 180 degrees phase difference superpose and cancel each other out
Define constructive interference
When two points on two waves with 360 degrees/0 degrees phase difference superpose and double the amplitude
What happens when a wave is reflected from a fixed point?
It changes direction and flips over
What happens when a wave is reflected from a loose point?
It is reflected but does not flip over
What is an interference pattern?
A graphical illustration of the superposition of two waves aka the pattern that emerges when two waves superpose
How is a standing wave produced?
When two waves with the same frequency and similar amplitudes moving in opposite directions superpose
Define a standing wave
A wave that is a superposition of two waves moving in opposite direction with similar amplitudes and the frequency
Give an example of how standing waves are used
They are used to produce notes on stringed instruments, when a string is plucked the string becomes a wave and it travels up and down in opposite directions to produce a standing wave
What is the relationship between the length and the wavelength in the 1st, 2nd, 3rd, 4th, 5th and 6th harmonic frequency
1st: Wavelength/2 = Length
2nd: Wavelength = Length
3rd: (3 x Wavelength)/2 = Length
4th: 2 x Wavelength = Length
5th: (5 x Wavelength)/2 = Length
6th: 3 x Wavelength = Length
Define an antinode
The part of the wave where the string moves the most
Define a node
The part of the wave where the string remains stationary
What is the equation for the speed of waves on a string?
c = _/T/u
Speed = _/Tension / Mass per unit length
What happens to the areas of constructive and destructive interference when you increase the distance between two wave sources that are producing waves that are superposing?
The areas get smaller and closer together
What are three things you can do to change the frequency of a note produced by a stringed instrument?
- Change the tension
- Change the mass per unit length
- Change the length
What is the relationship between the path differences when constructive interference occurs?
The path difference between two waves is multiples of a whole wavelength
What is the relationship between the path differences when destructive interference occurs?
The path difference between two waves is multiples of half a wavelength
What happens to the spacing of bright fringes if you increase the wavelength of the waves in double slit diffraction?
The bright fringes get further apart
How wide does the slit need to be in order for diffraction to occur?
It must be similar to the wavelength of the wave you want to diffract
What is the auditory effect of an interference pattern between light waves?
Areas with varying volume
What are the two conditions for coherency?
- Same frequency
- Constant phase difference
How do you make two waves monochromatic?
Pass them through a coloured filter
Why is it hard to get two waves with a constant phase difference?
Light sources produce discrete photons in a random manner which means that they all have different phase relationships
Describe what happens in Young’s double slit experiment
Light is passed through a coloured filter and then a single slit, this means that they are monochromatic and have a constant phase difference. Then these coherent waves are passed through a double slit which leads to an interference pattern being produced on the screen.
What happens to the bright fringes if you increase the wavelength of waves in single slit diffraction?
The spacing of the bright fringes increases
What happens to the bright fringes when you increase the slit width in single slit diffraction?
The spacing of bright fringes decreases
What does the intensity pattern for single slit diffraction look like?
The central bright fringe has a high intensity and subsequent bright fringes have a much lower and quickly decreasing intensity and are all half the width of the central bright fringe
What is the small angle approximation?
The idea that value of sin(x) and tan(x) are approximately the same as you tend towards 0 because the graphs follow the same pattern when dealing with very small values of x
Describe what happens when a wave progresses considering Huygens’ theory of wave propogation
The movement of particles between peaks and troughs means that they produce multiple wavelets, an infinite number of these wavelets is produced which then constructively interfere to form another wave front (peak)
Why is the single slit diffraction of light unexpected?
Since light travels as a particle it was not expected that light would diffract
If the first dark fringe appear at angle x where would the second dark fringe appear?
2x
When will bright fringes have the same intensity in double slit diffraction?
If the slit width is very small
What happens to the intensity pattern for single slit diffraction when you increase the slit width?
The intensity pattern becomes closer together
What happens to the intensity pattern for double slit diffraction as you move from a narrow slit to a wider slit?
The intensity decreases
What happens if you pass white light through a double slit?
You will see one central white bright fringe and then a spectrum
What is a diffraction grating?
A material that consists of multiple slits through which light can diffract
What are bright fringes caused by a diffraction grating called?
Orders
What determines the space between bright fringes with regards to diffraction gratings?
Wavelength
