6-Wave Behaviour Flashcards
What is the principle of superposition?
When 2 or more waves overlap, the resultant displacement at a given instant and position is equal to the sum of the individual displacements at that position.
What does the term ‘superpose’ mean?
Waves overlapping
What is the wavelength?
The distance between any 2 points on the same part of a wave cycle
What is the amplitude?
The maximum displacement from the equilibrium position
What can you use to measure the time period of a wave?
An oscilloscope
What equation relates time period and frequency?
Frequency = 1 / Time period
What does phase describe?
The stage in the wave cycle
What does the term ‘in phase’ mean?
When two points are at the same stage in the cycle
If point P is at the amplitude of a wave, and point Q is at the same amplitude of the wave, 2 wavelengths along, are the points in phase?
Yes
When 2 waves are ‘in phase’ what does this mean?
They are doing the same thing in the exact same moment
When 2 waves are in phase, is there any phase difference?
No, there is 0 phase difference
What does it mean when 2 waves are in antiphase?
They are doing the exact opposite things in the exact same moment
When 2 waves are neither in phase, nor in antiphase, what are they said to be?
Out of phase
How can phase and phase difference be measured?
By a phase angle
What is a phasor?
A rotating arrow which shows where the wave is in its cycle
How many radians does a phase arrow turn as the wave goes through 1 cycle?
2 pi radians
What does the vertical displacement of the clock arrow represent?
The displacement of the wave in that given instant and time
If 2 waves are in phase, what is the difference in phase angle?
0
If 2 waves are in antiphase, what is the phase angle difference?
pi radians
How can we calculate the displacement when 2 waves superpose?
We add the individual displacements together
When we add phasor arrows together, tip-to-tail, what can we find?
The amplitude of the resultant phasor
What is a ripple moving across the surface of water?
An example of a progressive wave-as you can see the crest of the wave moving
When 2 progressive waves move in opposite directions(e.g. along a string) what appears to happen?
The waves appear to stop moving
What is a standing wave?
When 2 progressive waves travel in opposite directions and it appears that the wave has stopped moving
When a string is plucked, what do waves do?
They move along the string in opposite direction, reflect at the end of the string, and superpose as they pass through one another
What is a node in a wave cycle?
A point where the waves travelling in a string meet in antiphase and there is 0 amplitude
What is the antinode in a wave cycle?
A point where the waves meet in phase causing a maximum displacement
What is the fundamental frequency?
The lowest frequency of vibration
What is the longest standing wave?
Twice the length of the string
What does wave velocity equal?
Wavelength X Frequency
Describe how standing waves can be formed in air:
Sound wave travels along a tube, where it reflects and waves travelling up and down the tube superpose with each other
Can sound waves be reflected from both the closed end and open end of a tube?
Yes
When is a node formed?
When waves meet in antiphase
With sound waves travelling along a tube, what is always formed at the open end of the tube?
An antinode
What is refraction?
When waves change speed when they change medium
What is a medium?
The material the wave is travelling through
When waves refract, what causes them to change directiomn?
The change in speed causes the light rays to bend and change direction
What is a vacuum?
A region of space which contains no matter
What is the speed of light in a vacuum?
3 X 10^8 ms-1
Why does light travel slightly slower in air than in a vacuum?
Because it interacts with the electrons in the atoms in the air
Why do electrons in atoms not cause light waves to slow down considerbly?
Because the air is not very dense, so interaction between light and the electrons are few and far between
What speed does light travel at through glass?
2 X 10^8 ms-1
Why does light travel much slower in glass than in air?
Because glass is much denser than air, so the number of interactions per metre between the light and the electrons is much higher
What is the refractive index?
The ratio of the speed of light in once medium compared to the speed of light in another medium
What is the first equation for the refractive index?
Refractive index = Speed of light in medium 1 / Speed of light in medium 2
If the first medium is a vacuum, what does the equation for the refractive index become?
Speed of light in vacuum / Speed of light in material
What is the normal?
An imaginary line at 90 degrees to the surface of the glass
When a ray of light travels from air into glass, what does the ray do?
It bends towards the normal
What is snell’s law?
sin i / sin r = Cmedium1 / Cmedium2
For Snell’s law, what is i and what is r?
i is the angle of incidence
When wave fronts enter glass from the air, what happens to the waves?
They slow down, as the wavelengths get shorter
What is the term interference used to describe?
The effect of the superposition of waves
What does the superposition of waves produce?
An interference pattern
When two waves meet in phase, will the sound be loud or quiet?
Loud
If the two waves from 2 different speakers are of the same amplitude when they meet in antiphase, what will ahppen?
They will cancel completely, producing silence
What is path difference?
The difference in distance travelled by two waves from their source to the pattern
When waves meet with zero path difference, are they in phase?
Yes
For what value of the path difference do waves meet in phase?
When the path difference = n X wavelength
For what value of the path difference do waves meet in antiphase?
(n+0.5) X wavelength
Two loudspeakers emit a wavelength of 10cm, suggest 2 values of path difference which will produce superposition maxima:
10cm, 20cm
Two loudspeakers emit a wavelength of 10cm, suggest 2 values of path difference which will produce superposition minima:
15cm, 25cm
What is a stable superposition pattern?
One which the position of the maxima and minima don’t change over time
When will stable superposition patterns only occur?
When there is a constant phase difference
What are coherent waves?
Waves with a constant phase difference
What are incoherent waves?
Waves that don’t have a constant phase difference
When does constructive interference occur?
When waves meet in phase, leading to large superposition amplitude
When does destructive interference occur?
When waves meet in antiphase, leading to lower superposition amplitude
When waves pass through a gap of roughly the same width as their wavelength, what do they do?
They spread out
What is diffraction?
Diffraction is when waves spread out after being passed through a narrow slit
What does the amount that wave spread out depend upon?
The width of the gap compared to the wavelength of the waves passing through
For a given wavelength passing through a gap, what happens as the gap gets narrower?
The greater the spreading caused by refraction
Does diffraction alter the wavelength, speed or frequency of waves?
No
Why do we not experience the diffraction of light in everyday life?
Because the wavelength is in the order of 10^-7, so is not diffracted often
What did Young’s double Slit experiment support the theory of?
That light travels as a wave
In Young’s double slit experiment, what is ‘d’?
The distance between the slits
For waves with a path difference of 1 wavelength, what does wavelength equal?
Wavelength = distance between slits X sin thet
For path difference of n X wavelength, what equation can we derive?
n wavelength = d sin theta
What does the order of maximum show?
The number of wavelengths path difference from two adjacent slits
For small angles, what does tan theta equal?
sin theta
What does path difference equal?
d X sin theta
As sin theta = x / L, what does path difference equal?
dx / L
As tan theta = x / L , and sin theta = lander/d, what equation can be derived from this?
wavelength = xd / L
What is x?
The spacing between the bright fringes on the screen
What is L?
How far away the screen is from the slits
What is d?
The distance between the slits
What is a diffraction grating?
A multiple slit version of the two slit system
Why does using a grating of many slits increase the brightness of the image on the screen?
Because more light gets through
Why are diffraction gratings able to spread white light into its component colours?
Because each wavelength of light will produce a maxima at a different angle
What does the line separation equal?
1 / number of lines per metre
Describe how in a slit experiment we get a maximum:
Where all the phasor arrows are in the same direction we get a large resultant, so an area of high intensity/ a maximum
Describe how in a slit experiment we get a minimum value:
The phasors add to zero, creating an area of zero intensity. The waves have (n+0.5)wavelength path difference, creating destructive interference
What is the first order of maximum?
The first bright image to either side
What is the second order of maximum?
The second bright image to either side
What does the term ‘coherent’ mean?
Two or more waves are said to be coherent if the phase difference between them is constant. Note that you can infer from this that if the waves are coherent, then they must have the same frequency and wavelength.
Why are a pattern of nodes / antinodes seen along string of musical instrument or column of air in tube or water on surface of pond etc?
Travelling wave and reflected travelling wave pass through each other in opposite directions, which is caused by constructive superposition at specific locations leads to antinodes, destructive superposition at other locations leads to nodes
Why are bright fringes observed at specific angles on a screen in a double slit experiment? (in terms of path length)
Path difference for wave trains passing through adjacent slits is an integer number of wavelengths for this angle(n lander), so the waves SUPERPOSE in phase / constructively to give bright spot / fringe
Why are bright fringes observed at specific angles on a screen in a double slit experiment? (in terms of phasors)
Phasors add to give large resultant amplitude to give bright spot / fringe
Why are dark fringes observed on screen in double slit experiment or diffraction grating experiment at a particular angle? (in terms of path length)
Path difference for wave trains passing through adjacent slits is an ODD number of HALF wavelengths for this angle, so waves SUPERPOSE in antiphase / destructively to give dark spot / fringe
Why are bright fringes observed at specific angles on a screen in a double slit experiment? (phase explanation)
Waves in phase as they pass through the slits; in phase when they reach the screen as the phase difference between them is integer multiple of 2π radians, so waves SUPERPOSE in phase / constructively to give bright spot / fringe
Why are dark fringes observed on screen in a double slit experiment or diffraction grating at a particular angle? (in terms of phase)
Waves in phase as they pass through the slits; out of phase when they reach the screen as the phase difference between them is an ODD multiple of π radians (π, 3 π, 5 π etc.), so the waves SUPERPOSE in antiphase / destructively to give dark spot / fringe
Why do waves spread out when passing through a single slit?
At one particular angle, path lengths for set of n wave trains across the slit differ by λ/n, Waves SUPERPOSE in antiphase / destructively: this defines the edge of the beam
Why is there a spectrum of colours produced by light falling on oil film / soap bubbles?
Path difference for waves reflected from front and back surface of film / bubble is an integer number of wavelengths FOR ONE PARTICULAR COLOUR / ANGLE, so waves superpose in phase constructively for one colour and angle of light hitting film / bubble, leading to this colour being seen brightly in reflection
Why does narrower slit spacing / grating line spacing produces wider spread of fringes / spots?
nλ = dsinθ, so if d is reduced then sinθ must increase as nλ is unchanged, so angles at which bright / dark spots / fringes seen is increased.
Why is there a multiple spectra of colours produced when white light shone through diffraction grating, but straight-through beam remains white?
nλ = dsinθ, so for straight-through beam n=0, so sinθ = 0 for all colours, so see white as all colours superimposed. For n=1 and higher orders, red light is diffracted more than violet light, as red has longer wavelength, so red beam seen at larger angle than violet for each order.
What is the limitation of the wave model of light?
Cannot explain photoelectric effect. Wave model predicts that number of photoelectrons emitted per second should depend only on the light intensity, not its frequency.
Cannot explain random build-up of intensity in a photographic image. Wave model predicts that there should be the same variation of intensity initially as in the final image
What is the limitation of the photon model of light?
Cannot explain interference phenomena, as two photons, being particles, cannot add together and cancel each other out.
