Waves Flashcards
What is a wave
A physical phenomenon that transfers energy through a medium without transfering matter
How do waves travel and transfer energy in a medium
Through oscillations
What is a transverse wave
Wave in which the direction of oscillation is perpendicular to the direction of wave travel
What is a longitudinal wave
Wave in which direction of oscillation is parallel to the direction of wave travel
Define Displacement
Distance and direction of a vibrating particle from the equilibrium posituon
Define Amplitude
Maximum of a vibrating particle from the equilibrium position
Define Wavelength
Distance between two adjacent vibrating particles with same velocity at the same displacement
Define Period
Time taken for a particle to complete one oscillation
Define frequency
Number of complete oscillations performed per second by a particle
Frequency Formula
f = 1 / T
What property of a wave is constant as it travels through a medium
Wavelength
What changes as a wave crosses between media
Speed and Wavelength
Wave equation
v = f x wavelength
Define Phase
Fraction of a complete wave that a particle is at
Define Phase Difference
The difference in the phase between two points along the same wave or between two waves at any given point in time
Phase difference for two waves in phase
Even integer of pi
Phase difference for two waves out of phase
Odd integer of pi
Define Reflection
When a wave reverses direction upon meeting the boundary between two different medium
Law of Reflection
Angle of Incidence = Angle of Reflection
Define Refraction
When a wave changes direction upon crossing the boundary between two different media
What does refraction lead to
A change of wavelength
What happens as waves slow down
Bend towards normal
Wavelength gets shorter
What happens as waves speed up
Bend away from normal
Wavelength gets longer
When do sound waves speed up
Going into physically denser mediums
When do EM waves slow down
Going into optically more dense media
Define Diffraction
The physical phenomenon of waves spreading out when passing through a gap, or around an obstacle
Narrower the gap the diffraction is?
Greater
Longer the wavelength, the diffraction is?
Greater
What must happen for significant diffraction to occur
Gap size has to be of same order of magnitude as wavelength
What doesn’t change upon diffraction
Wave speed and wavelength
Define Polarisation
Property of transverse waves which defines the plane of oscillation of the wave
Plane of polarisation definition for an EM Wave
The plane in which the electric field vibrates
What kind of EM waves do most sources of light generate
Unpolarised EM Waves
What does unpolarised light consist of
Wave-trains within which different waves have their E-field aligned in different planes
2 ways EM waves can be polarised
Absorption and reflection
When the plane of polarisation of the incident wave is at some angle to the plane of alignment of the filter, what relationship do we use for intensity
Intensity is directly proportional to cos^2 (theta)
What filters do sunglasses have and why
Vertically aligned polaroid filters
Block all horizontally plane polarised light
When is a wave polarised
When vibrations occur in a single plane
When is a transverse wave unpolarised
Vibrations change from one plane to another
Define Intensity
Power per unit area
What happens to waves from a point source as they spread out
They travel outwards
Intensity falls as power of source is spread over an increasing area
Area over which power is spread = surface area of a sphere of radius equivalent of the distance travelled from the source
Intensity at a given distance from the source of power is given by
I = P / A
I = P / 4 pi r^2
What relationship does intensity of a wave have with distance travelled from source
Inverse square relationship
Relationship between Intensity and Amplitude
Intensity is directly proportional to amplitude squared
Wavelength for Radio waves
> 10^-1
Wavelength for microwaves
Between 10^-3 and 10^-1
Wavelength for Infrared waves
7 x 10^-7 to 10^-3
Wavelength for visible waves
4 x 10^-7 to 7 x 10 ^-7
Wavelength for UV Waves
10^-8 to 4 x 10^-7
Wavelength for X rays
10^-13 to 10^-8
Wavelength for gamma rays
<10^-13
Range of wavelengths where x rays and gamma rays overlap
10^-13 to 10^-10
When do all EM waves travel at the same speed
Only through a vacuum
For EM waves how does the electric field vector oscillate
90 degrees to the direction of wave travel
Define Refractive index
ratio between the speed of light in a vacuum to the speed of light in that medium
n = c/v
What happens with a greater refractive index
Greater the decrease in speed in a medium and the more light refracts in the medium
Define TIR
Wave phenomenon by which light completely reflects back at a boundary between two media
Conditions for TIR
Medium within which light is incident has a larger refractive index than the second medium
Angle of incidence exceeds the critical angle
Define the principle of superposition
When two waves meet, the total displacement at a given point in time is the vector sum of the two individual displacementa
What occurs when waves meet in phase
Constructive interference
What happens when waves meet in antiphase
Destructive inteference
What is Interference
Effect that is observed upon the superposition of waves
Doesn’t always occur when waves superpose
When are interference effects observed in practice
When two coherent wave sources superpose
Define coherent wave sources
Wave sources that have the same frequency and constant phase difference at a given point through time
What is the P.L.D between two waves
Difference in length in the paths travelled by each wave
What does PLD give rise to
Phase Differences
What happens with PLD with an even multiple of wavelength / 2
P.D is multiple of 2pi
Constructive interference
What happens with PLD with an odd multiple of wavelength / 2
P.D is odd multiple of pi
Destructive interference
Youngs Double Slit Experiment Method
Light from a lamp was passed through a filter
Produces a monochromatic source of light
Monochromatic light was then made incident on a single slit - made to diffract and used to illuminate a double slit - produces two sources of coherent waves
As the light waves from each double slit move forward - they superpose - producing dark and bright interference fringes at a screen - Lights interference pattern
Youngs Double Slit Equation
only holds true when a «_space;D
wavelength = slit separation x fringe separation / distance between slits and screens
ax/D
When do stationary waves form
When two progressive waves of the same frequency travel in opposing directions and superpose
For given systems - only certain frequencies travelling along the system will produce stationary waves
What is the first harmonic / fundamental frequency
Simplest stationary wave that can be produced
Lowest frequency sound that can be produced on a string of a given length, mass and tension
1 antinode in the middle 2 nodes at both ends
L of string = lambda / 2
lambda = 2L
f0 = v /2L
What is the second harmonic
Second mode of vibration that can be produced
2 antinode in the middle 3 nodes
L = lambda
lambda = L
f1 = v / L
f1 = 2f0
What is the third harmonic
Third mode of vibration that can be produced
3 antinode in the middle 4 nodes
L = 3lambda / 2
lambda = 2L/3
f2 = 3v/2L
What are Photons
Packets of discrete EM energy
Energy of a photon equation
E = hf = hc/lambda
What is the UV catastrophe
Wave theory can not explain the existence of peaks in intensity at particular wavelengths - as intensity of radiation from an object should become infinite at smaller and smaller wavelengths
How did Planck solve the UV Catastrophe
Introduced the idea that energy of vibrating atoms can only be in multiples of a basic amount
(quantised)
Introduced Planck’s Constant in E=hf
What does a laser beam consist of
Photons of the same frequency
Power of beam equation
nhf
n is the number of photons passing a point every second
What is the Electron Volt
Unit eV
Work done on/by an electron in accelerating it between a potential difference of 1 volt
V = E/Q
E= VQ
E = Ve (for an electron of e moving across a p.d V)
Number of eV equation
Energy in J / 1.6 x 10^-19
What can be assumed at threshold p.d of a diode
Energy of a single electron is transferred completely to the energy of a single photon of a given frequency/wavelength
What is the Photoelectric effect
The emission of electrons from the surface of a metal, when it is illuminated by EM radiation above a certain threshold frequency
Photoelectric Effect Observations
Emission of electron only occurs if frequency of incident EM radiation is above threshold
Incident EM radiation above the threshold frequency results in instantaneous emission of electrons
Increasing the intensity of incident radiation increases the number of electrons per second not KE - as long as its above threshold frequency - KE can only be increased by increasing the frequency of incident radiation
Photoelectric Effect Explanations
A single electron on a metal absorbs the energy carried by a single photon
If the energy carried by a single photon exceeds the work function of the metal - the electron is able to escape the metal
Any energy in excess of the work function absorbed by the electron becomes its kinetic energy
hf = E(kmax) + work function
Emission only takes place is hf > work function
Increasing intensity of incident EM radiation increases number of photons passing per unit time - increasing the number of photon-electron interactions per unit time - increasing the number of emitted electrons
Idea of KEmax
Relative position of electrons within a metal dictate how much energy is required to free them from a given metal
Electrons on the surface on a metal are subject to fewer electrostatic forces of attraction to ions - and thus require the least amount of energy to free
For any given frequency of incident radiation above threshold - only few, surface electrons acquire max KE
Work function is only for surface electrons
When is wave like nature of light observed
During diffraction - interference patterns
Light emerging from a narrow slit spreads out - superposes and forms an interference pattern
When is particle like nature of light observed
Photoelectric effect
Light above a certain threshold frequency is incident on a metal surface - an electron will absorb a single photon
Energy of the photon is a function of the frequency of the light
If the energy of this photon is not greater than the work function of the metal - the electron will not escape
What does the de Broglie Wavelength equation tell us
Th wavelength of a particle is equal to its momentum
De Broglie Equation
lambda = h/p
p = particles momentum
Wavelength of a particle depends inversely upon its mass and velocity
What happens to de Broglies wavelength when speed increases
It decreases
Electron Diffraction and Wave Particle Duality
Electrons can only be diffracted by metal lattices where the atomic spacing between is comparable to the size of the electron de Broglie Wavelength
Electrons are only diffracted at certain angles - formation of rings
KE and velocity of the electrons can be increased by increasing the accelerating potential between the filament node
