Waves Flashcards
Continuous Wave definition
Constant amplitude and frequency
Wavelength definition
The distance between any point and the next identical point
Frequency definition
Number of oscillations in one second
Measured in Hz
Frequency and Time equation
f = 1/T
Speed/Frequency/Wavelength equation
c = f * wavelength m/s = Hz * m
Dispersion definition
Waves separate out whilst travelling through a medium
Due to different wavelengths travelling at different speeds
Displacement definition
Distance from the centre of the oscillation to the particle
Vector, so can be negative
Amplitude definition
Maximum displacement
Energy/Amplitude equation
Energy is directly proportional to amplitude squared
E = k A^2
Out of phase definition
Two waves with same frequency and wavelength, but one is shifted compared to the other
Single slit diffraction pattern
Central maxima
Decreasing height of maxima as you get further from centre
Double slit interference
How to increase fringe separation
Distance from slits to screen is increased
Wavelength of light used is increased
Slit spacing (distance between centres of slits) is decreased
Fringe separation equation
w = λD/s w = fringe separation λ = wavelength of light D = distance from slits to screen s = slit spacing
Slit spacing definition
Distance between centres of slits
Why are fringes formed
Interference of light
Why are bright fringes formed from double slit
Light waves from each slit are in phase
Path difference = lambda
Why are dark fringes formed from double slit
Light waves from each slit arrive 180º out of phase
path difference = lambda/2
Why are lasers used in double slit experiments
Coherent source of light => no need for a single slit before.
Photons in a laser beam are in phase with each other
Highly monochromatic, meaning its wavelength can be calculated to within a nanometre
What happens when white light is used for double slits
Central fringe is white
Each component colour of white light produces its own fringe pattern
Each pattern is centred on the screen at the same position
Principle of superstition
Exact moment when 2 waves meet Crest + crest = super crest Trough + trough = super trough Crest + trough = 0 displacement - If waves aren't of equal magnitude, crest + trough = minimum
In phase definition
Two waves superimpose constructively
Anti phase definition
two waves superpose destructively
Node definition
Resultant wave = 0 displacement
No oscillation
Anti-node definition
Resultant waves has largest amplitude
Maximum oscillation
Distance between nodes
= distance between antinodes
= wavelength/2
Difference between transverse and longitudinal waves
Transverse:
vibrations act perpendicular to direction of energy transfer
Longitudinal:
vibrations occur parallel to direction of energy transfer
How is light proven as a wave?
Interference
- Used if the size of the object is comparable to the wavelength of the light
Or diffraction and polarisation
How is light proven as a particle?
Photo-electric effect
- Used if the energy is comparable to the energy of the photon (E=hf)
Which experiment(s) prove light could be both a wave and a particle?
Reflection
Refraction
Emission vs absorption
- Hot object will emit a continuous spectrum of light
- Cool gas will absorb some of the photons with specific energies, exciting the orbital electrons
- Orbital electrons will return to the ground state and emit photons with the same frequency
Forced vibrations definition
Vibrations/oscillations of a system subjected to an external periodic force
Resonance definition
System vibrates such that its velocity is in phase with the periodic force
An oscillating body that is not resonating will return to its natural frequency when the forcing vibration is removed
Damped oscillations definition
Oscillations that reduce the amplitude for ALL frequencies due to the presence of resistive forces such as friction and drag
- Lightly dampened system, amplitude of oscillations decreases gradually
- Heavily dampened system displaced from equilibrium then released, the system slowly returns to equilibrium without oscillating
- Critically damped system, it returns to equilibrium in the least possible time without oscillating
As an applied frequency to a system increases…
- The amplitude of oscillations increases until it reaches a maximum amplitude at a particular frequency, and then the amplitude decreases again
- Phase difference between DISPLACEMENT AND THE PERIODIC FORCE increases from 0 to pi/2 at maximum amplitude, then from pi/2 to pi as the frequency increases further
Resonant frequency definition
The frequency of a system at the maximum amplitude
For an oscillating system with little/no dampening, at resonance…
The applied frequency of the periodic force = the natural frequency of the system
Diffraction grating definition
A plate with may closely ruled parallel slits on it
How is the angle of diffraction between each transmitted beam and the central beam increased (diffraction grating)
Light with longer wavelength used (e.g. blue filter instead of red filter)
Grating with finer slits is used
Diffraction grating equation
d * sin theta = n * lambda
d = slit separation (number of slits/metre = 1/d) theta = angle between central maxima and nth order n = order number lambda = wavelength of light used
What does an increase of the number of slits do (diffraction grating)
Narrower peaks
More precise measurements
Increased intensity
Higher concentration of light
Electromagnetic spectrum
Smallest frequency => highest frequency
Largest wavelength => shortest wavelength
Radio waves - microwaves - Infared - visible - UV - X-Ray - Gamma Ray
Visible light:
Violet shortest wavelength, red longest
BLUE HAS SHORTER WAVELENGTH THAN RED
Types of spectra
Continuous
Shows all colours
Most intensive part of the spectrum depends on temperature
- Hotter the light source, the shorter the wavelength of most intensive part
Means that by measuring the wavelength of most intensive part, we can measure the temperature of the light source
Line emission
Glowing gas in a vapour lamp/discharge tube emits light at specific wavelengths
Characterised by the chemical element that produced the light
Line absorption
Continuous spectrum with narrow dark lines at certain wavelengths
Due to elements in glowing gas, which absorb light of the wavelengths they can emit, so the transmitted light is missing these wavelengths
Young’s double slit
How is the pattern formed on the screen?
Each slit diffracts light, emitting waves
Two slits act as coherent sources of light
Proves light is a wave as interference is a wave property
Bright fringes formed when light from one slit reinforces light from the other
Light from slits is in phase at a maxima, causing reinforcement
Path difference to bright fringe = n lambda (whole number of wavelengths)
Path difference to a dark fringe = (n+1/2) lambda (half wavelengths)
