U2.4 Superposition, Interference & Diffraction Flashcards
(23 cards)
What is the principal of superposition?
When two or more waves meet at a point the resultant displacement is equal to the vector sum of the individual displacements due to each wave at that point.
What happen when two waves overlap in phase?
They produce a wave of greater amplitude.
This is called constructive interference.
If both waves crests coincide exactly (and troughs). The amplitude is doubled.
What happens if the crest of one wave coincides with the trough of another wave (anti-phase)?
The waves cancel out each other if the amplitudes are equal.
This is called total destructive interference.
If the amplitudes aren’t the same then then destructive interference takes place and the resultant wave will have an opposite amplitude.
What does it mean if two waves are coherent?
The sources must produce waves of the same wavelength or frequency and be in phase, i.e. each produces a crest at the same time or have constant phase difference between them.
What is the other name for stationary waves?
Standing waves.
When do standing waves occur?
When waves of equal amplitude and equal wavelength are travelling with the same speed in opposite directions.
What happens to the energy in a standing wave?
The energy is not transferred but instead stored in the wave.
When does resonance occur?
When the driving frequency matches the natural frequency of the stretched string.
What are nodes and anti-nodes?
A node is any point on a standing wave where displacement is zero.
An anti-node is the opposite, they are any point where displacement is maximum.
Describe the fundamental frequency.
It has one loop.
λ = 2L
v = ƒ0 x L
Describe the first overtones.
It has two loops.
λ = L
v = 2ƒ0 x L
Describe the second overtone.
It has three loops.
λ = 2L ÷ 3
v = 3ƒ0 x 2L ÷ 3
What is the definition of phase difference?
The difference in phase of two waves at a point.
What is the definition of path difference?
The difference in distance travelled by waves from two sources to a given point.
What is a path difference of one λ equal too in phase difference?
360º
How can you tell if path difference is constructive or destructive?
If path difference is a whole number of λ then it is constructive interference.
If it’s not a whole number of λ then it id destructive interference.
What must happen to produce an observable interference?
Both sources must have a constant phase difference.
They must be coherent sources.
How is the condition for constructive and destructive interference written?
Constructive = nλ
Destructive = (n+1/2)λ
What is Youngs formula?
λ = a x y ÷ d
λ = wavelength
a = separation of the double slits in m
y = fringe separation on the screen in m
d = distance from the double slit to the screen in m
What is diffraction?
The spreading out of waves passing through an aperture or passing by an obstacle.
When does maximum diffraction occur?
When the gap width is approximately equal to the wavelength of the wave.
What happens to wavelength as a result of diffraction?
Nothing.
What is the equation for diffraction grating?
d x SinØ = n x λ
d = the grating spacing in m
Ø = the angle of diffraction in degrees
n = the diffraction order maximum
λ = the wavelength of light in m
