Chapter 12 Flashcards
What is superposition?
A physical phenomenon where 2 waves meet and they overlap so that their displacements combine
When 2 wakes meet, the total displacement at a given point in time is the vector sum of the 2 individual displacements
What happens when 2 waves continually pass through each other?
They superpose which creates a resultant wave
What dictates the final magnitude of the superposed wave?
Phase difference and path length difference
When does constructive interference occur?
When 2 waves that are in phase(even integer of pi) meet so the resultant displacement is of a higher displacement
Remember that intensity is proportional to amplitude^2
When does destructive interference occur?
When 2 waves meet in antiphase(odd integer of pi), the max positive displacement adds to the max negative displacement so the resultant displacement is lower
What happens if both waves have the same amplitude and they are in anti phase?
Resultant displacement is 0
What is interference? When is it observed?
The effect observed when 2 or more waves superpose
Only observed when 2 COHERENT wave sources superpose
What is a coherent wave source?
A wave source that maintains a constant phase difference throughout time and have the same frequency
What are the 2 conditions for coherency?
Temporal coherency: waves have to be the same frequency
Spatial frequency: each source maintains the same displacement with regard to each other
When can interference patterns of light only be observed?
When 2 coherent sources superpose
If the phase difference changes in time, it would be seen as a constant line
What happens at maxima and minima?
At maxima, the waves interfere constructively
At minima the waves interfere destructively
What is path length difference?
The difference in length in paths traveled by 2 difference waves. It is measured as a fraction of wavelengths
What is constructive interference and destructive interference
Constructive: when path length difference is an integer of lamda and waves arrive in phase
Destructive: when path length difference is an odd multiple 1/2 of lamda like 1 half, 3 half
What does Young’s double slit experiment show?
Light acts as a wave
What is Young’s experiment?
Light from a lamp is passed through a colour filter so light is monochromatic (has the same frequency) and same wavelength
Light is passed through a single slit so it diffracts. It then arrives and passes through a double slit which produces 2 coherent wave sources that are in phase
Light miles forwards and interferes with each other which produces an interference pattern on a screen with bright and dark fringes
What cause the bright fringes?
Constructive interference where light arrives in phase. Light from one slit overlaps with each other and superposes
What causes the dark fringes?
Where light from one slit arrives in antiphase to light from the 2nd slit so they destructively interfere
What is the formula for wavelength in this experiment?
Wavelength = slit separation x fringe seperation / distance between slit and screen
What happens if you use a diffraction grating?
The pattern is really sharp because there are more beams interfering constructively. This means that fringes are also sharper
What is the other formula for this experiment?
Distance between slits x sin(angle between incident angle and angle of maxima) = order(of maxima) x wavelength
What does larger wavelength and smaller grating separation mean?
Larger wavelength means pattern spreads out more
Smaller grating means that there is less spreading out
What happens if you shine white light instead of monochromatic light?
There is a colourful pattern because different wavelengths spread out different amounts
What is a stationary wave?
A wave that oscillates in time but does not transfer any net energy
When are stationary waves formed?
Formed from the superposition of 2 progressive waves of the same frequency and wavelength which propagate in opposite directions
What is a node and an anti node?
A node is a point of 0 displacement where amplitude and intensity = 0. Also a point of destructive interference
An anti node is a point of maximum displacement where amplitude is max. Also a point of constructive interference
What is the desperation between 2 adjacent nodes or anti nodes?
1/2 lamda
What is the phase difference between adjacent nodes?
- Particles oscillate in phase between adjacent nodes because when one particle hits maximum displacement, so do all of them
What is the phase difference in different sides of a node?
Pi radians is the phase difference
Different sides of a node are in antiphase
Differences between progressive and stationary wave?
Progressive transfers energy in direction of propagation but stationary doesn’t
Progressive wavelength is the distance between 2 points oscillating in phase. Stationary wavelength is the distance between 2 adjacent nodes x 2
All parts between nodes are in phase, on opposite sides of nodes are in antiphase
Stationary waves have max amplitude at anti node and none at node. Progressive waves have max amplitude everywhere
What is the fundamental frequency?
The lowest resonant frequency (aka first harmonic)
Which factors determine fundamental frequency?
Mass, tension and length
What happens in an closed pipe?
The open end is an anti node and the closed end is a node
The length, density and temperature deter jong the fundamental frequency and wave speed
Only odd harmonics(odd multiples of the fundamental frequency) can form
Sound waves form stationary waves when reflected
What happens in an open air pipe?
Both ends are open so both have anti nodes
Density, temperature, length determine fundamental frequency and wave speed
Each harmonic is a multiple of the fundamental frequency
