AS2 2.4 Superposition, interference and diffraction Flashcards
What does it mean by Coherency?
Two waves are said to be “coherent” if they are the same type, have the same frequency and thus the same constant phase difference
Coherent waves can be in phase or out of phase. The phase of incoherent waves will change constantly
What is the Principle of Superposition?
When two or move waves meet, the resultant displacement of the medium at that point in space is equal to the vector sum of their individual displacements at that point
How does Constructive Interference occur?
If both waves are in phase / have the same sign displace ( i.e both positive or negative) constructive interference (reinforcement) will occur
How does Destructive Interference occur?
If at the point of overlap, waves are in antiphase , total destructive interference (cancellation) will occur
If one wave has a positive displacement and the other has a negative displacement, but they aren’t exactly in antiphase, the waves do not completely cancel out, but the resultant wave will have a smaller amplitude
Constructive Interference Summary
- Path Difference = whole number of wavelengths
- Phase Difference = 0° / 360° / 2π radians
- When the waves are in phase
(Total) Destructive Interference Summary
- Path Difference = odd number of half wavelength
- Phase Difference = 180° / π radians
- When the waves are in antiphase
What is a Standing Wave?
It is a wave in which no energy is transferred, but stored within the vibrating particles
What is a Progressive Wave?
It is a wave in which energy is transferred from one place to another
How is a Standing Wave formed?
Standing waves are set up when two waves meet that have the same frequency, speed and amplitude and are travelling in opposite directions superpose
Where are Nodes created?
A node is created where destructive interference occurs
Where are Antinodes created?
An antinode is created where constructive interference occurs
Properties of Stationary Waves
Nodes are points of 0 displacement and have minimum amplitude
Antinodes are points of maximum displacement and amplitude
A wavelength is equal to the distance between two nodes multiplied by 2
Standing Waves Speed
The speed of the standing waves remains the same for all modes of vibration
What is a Resonance?
Resonance is a condition where an object vibrates at its natural frequency and the amplitude of vibration is maximum
Conditions for Observable Interference
For observable interference to occur, the following conditions must be met for both sources:
1. They are Coherent
2. They have a constant phase difference
3. They have similar amplitudes
4. Light Waves only: must be monochromatic
What does Monochromatic Light mean?
Monochromatic Light is light of a single wavelength, i.e one colour
( remember that different colours of visible light have different frequencies/wavelengths )
Double Slit Experiment Equation
λ = ay / d
λ = light wavelength / m
d = distance from double slits to screen / m
a = slit separation / m
y = interference fringe separation / m
All distances must be in meters before doing the calculation
What are diffraction gratings?
A diffraction grating is a glass/perspex material with many parallel equally spaced opaque lines.
Using diffraction grating provides more slits, which increases the interference between the beams
Diffraction Gratings Equation ( number of lines)
number of lines per meter = 1 / distance between grating slits
Diffraction Gratings Equation ( sin )
dsinθ = nλ
d = spacing between slits (m)
θ = angle between maxima and central maximum (degrees)
n = order number of maximum
λ = wavelength of light
Maximum number of orders equation
n = d / λ
d = spacing between slits (m)
λ = wavelength of light
n always needs rounded down to the nearest whole number
What is Diffraction?
Diffraction is the spreading out of waves when they pass through an aperture/round an obstacle
Amount of Diffraction
The amount of diffraction increases as the gap gets smaller. Maximum diffraction occurs when the gap size = wavelength of the wave
