4. Waves Flashcards
Define displacement
Distance of an oscillating particle from its equilibrium position in a specified direction
Define amplitude
The maximum displacement of an oscillating particle from its equilibrium position
Define wavelength
The distance between any 2 successive points in phase
Define period
Time taken for a particle to undergo 1 complete cycle of oscillation
Define frequency
No. of complete cycles of a particle per unit time
Define wave speed
Distance the wave profile moves per unit time
Define transverse waves
Waves in which the displacement of the particles of the medium is perpendicular to the direction of the wave motion
Define longitudinal waves
Waves in which the displacement of the particles of the medium is parallel to the direction of the wave motion
What are electromagnetic waves?
2 sinusoidal fields (E & B field), oscillating in phase and perpendicular to each other
State all electromagnetic waves, in ascending order of wavelength
Gamma rays, X-rays, ultra-violet radiation, visible light, infra-red, microwaves, radio waves
Wavelength of gamma rays
10^-14
Wavelength of X-rays
10^-10
Wavelength of ultra-violet radiation
10^-8
Wavelength of visible light
10^-7
Wavelength of infrared radiation
10^-5
Wavelength of microwaves
10^-2
Wavelength of radio waves
10^1
Define wavefront
A line joining the points of a wave that are in phase
Define ray
The path taken by the wave
Define intensity
The rate of energy flow per unit cross sectional area perpendicular to the direction of wave propagation
Power/Area
How is intensity related to amplitude and radius?
Proportional to the square of amplitude
Proportional to 1/r^2
State the principle of superposition
When 2 or more waves of the same kind exist simultaneously at a point in a medium, the resultant displacement of waves at a given point in time and space is the vector sum of the displacement due to each wave acting independently.
What is the polarisation of light?
When its electric fields oscillate in a single plane, rather than any direction perpendicular to the direction of propagation
What is Brewster’s Law?
If the reflected ray and refracted ray are at right angles to one another, then the reflected ray is totally plane-polarised
What is Malus’s Law?
Measures the intensity of the polarised light beam after travelling through the polarizer
What happens to a wave which hits a fixed end?
It will be reflected in an inverted way
If it was a pulse going up there would be a pulse going down
What happens to a wave which hits a free end?
It will go up and then go in the opposite direction (visualise this since I can’t really explain this)
What happens when a wave encounters a boundary? (less dense –> more dense)
(1) Pulse will be reflected back in the opposite direction in an inverted way with 180 deg phase change
(2) Pulse will be transmitted on to the more dense medium with no phase change (more intense)
What happens when a wave encounters a boundary? (more dense –> less dense)
(1) Pulse will be reflected back in the opposite direction with no phase change (more intense)
(2) Pulse will be transmitted on to the less dense medium with no phase change
What changes when a light ray enters another medium?
Wave speed changes –> wavelength changes
State Snell’s law
n1/n2 = sin 2/sin 1
What is the critical angle?
The angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90 deg.
What happens when the angle of incidence is increased beyond the critical angle?
Total internal reflection takes place
State 2 conditions for TIR
- Ray of light travels from optically denser medium to less dense medium
- Angle of incidence in the optically denser medium is greater than crit angle
Define diffraction
The phenomenon of bending or spreading of waves when they pass through an obstacle or aperture
When will there be less or no diffraction (there’s still an obstacle)?
When the aperture size is too big compared to the wavelength
When will there be greater diffraction? Why?
When the aperture size gets closer to the wavelength. When wavelength is greater or equals to aperture size, the wave acts like a point source and spreads out like a single ripple in water.
Define interference
When waves from 2 or more coherent sources superpose with one another producing a resultant wave
What does it mean when waves are coherent?
When waves have constant phase difference –> same omega, same wavelength, same speed, same frequency
What is the maxima and the minima?
Max: Lines of constructive interference, where resultant amplitude = 2x amplitude of one wave
Min: Lines of destructive interference = 0 resultant amplitude
When does constructive interference happen? What is the path difference?
When 2 waves arrive in phase at one point. PD will be 0 or differ by an integral multiple of wavelength
When does destructive interference happen? What is the path difference?
When 2 waves arrive exactly out of phase at one point. PD will be (n + 1/2) wavelength, where n is an integer
What conditions must be fulfilled for 2 sources to produce an observable interference pattern? [3]
- Coherent
- Roughly same amplitude
- Either unpolarised or have the same plane of polarisation
What is a condition that must be fulfilled in order for all the fringes to be equally bright
When slit width is very small compared to the wavelength –> Becomes like there are 2 point sources
What are the 4 conditions for standing waves?
- Same type of wave
- Same amplitude
- Same frequency
- Travelling in opposite directions
Define ‘nodes’ and ‘antinodes’
Node: Points that show no displacement
Antinodes: Points that show maximum displacement
What is the 1st harmonic like when both ends of the vibrating string are fixed?
N-AN-N
The reflected wave is inverted and cancels out at the ends, therefore forming a node
What is the 2nd harmonic like when both ends of the vibrating string are fixed?
N-AN-N-AN-N
What is the 1st harmonic like in a vibrating air column?
AN-N-AN
Wave travels from higher pressure/density to lower pressure/density, so the less dense air outside acts as a soft boundary. The wave gets reflected back without being inverted & superposes at the ends of the air column.
What is the 2nd harmonic like in a vibrating air column?
AN-N-AN-N-AN
What is the 1st harmonic like in a tube closed at one end?
closed at the right side
AN-N
Superimposes on the left side without a fixed barrier, and cancels out on the right side with a fixed barrier.
What is the 2nd harmonic like in a tube closed at one end? (closed at the right side)
THERE’S NO 2ND HARMONIC HAHA PSYCH
What is the progression of harmonics like in all the 3 examples?
- Fixed at both sides
- Tube open at both sides
- Tube open at one side
- 1,2,3,4,…
- 1,2,3,4,…
- 1,3,5,7,9,…
What’s the 3rd harmonic like in a tube open at one end? (open at left side)
AN-N-AN-N
What’s the diff. between a stationary wave and a travelling wave in terms of energy?
Stationary: Energy not transmitted by wave, but energy is associated with it
Travelling: Energy is transmitted by wave
What’s the diff. between a stationary wave and a travelling wave in terms of amplitude?
Stationary: All points have diff. amplitudes
Travelling: All points have same amplitudes
What’s the diff. between a stationary wave and a travelling wave in terms of phase?
Stationary: All points between 2 nodes have the same phase (phase diff = 0)
Travelling: All particles along a wavelength have different phases
What is a progressive wave?
A wave in which energy is transferred from particle to particle as the wave propagates.
