5 - Waves and Particle Nature of Light Flashcards
Define oscillation
Motion that repeats at regular intervals (periodic oscillation/harmonic motion)
One wave cycle = ?
One oscillation (2π/360°)
Define wave phases
They describe the stage of a wave’s oscillation
What is the phase difference when waves are in antiphase?
π (180°)
What is the phase difference when waves are said to be ‘in phase’?
They are at the same stage, so 0
What is the phase difference when waves are said to be in quadrature?
π/2 (90°)
f = ?
1/T
Transverse waves have oscillations ___ to its propagation?
Perpendicular
Longitudinal waves have oscillations ___ to its propagation?
Parallel
What does the microphone in the oscilloscope traces core practical do?
Uses small movements of its diaphragm to produce an electrical signal that mimics variation of pressure/displacement in a wave
What happens to amplitude when two waves (with same amplitude) are in phase?
Amplitude doubles (constructive intereference)
What happens to amplitude when two waves (with same amplitude) are in antiphase?
No amplitude (destructive interference)
Conditions for 2 waves to be coherent when in phase
-All peaks/troughs line up
-Waves have same amplitude and wavelength
-Constant frequency
Conditions for 2 waves to be coherent when they are out of phase
-Consistent phase difference
-Same amplitude and wavelength
-Constant frequency
Define standing wave
Due to the superposition of 2 waves with the same amplitude and frequency traveling at the same speed but in opposite directions
What is the phase change of a standing wave at the fixed end?
π (180°)
Define node
Points of 0 displacement on a standing wave
Define antinode
Points of maximum displacement on a standing wave
Distance between 2 nodes
λ/2
Distance between a node and antinode
λ/4
How does mass and tension affect frequency on a plucked string?
-Tension increase = frequency increase
-Length decrease = frequency increase
-Mass per unit length decrease = frequency increase (thinner string)
Define diffraction
The spreading out of waves when they pass through a gap/around an obstacle, especially when the size of the gap/obstacle is comparable to the wavelength of the waves
Define superposition
The principle that when 2 or more waves overlap, the resultant displacement at any point is the sum of the displacements of individual waves at that point
Define interference
The phenomenon that occurs when 2 or more waves meet and combine, resulting in a new wave pattern
Define constructive interference
In phase, increasing amplitude
Define destructive interference
Out of phase, reducing/nullifying the resultant wave
Equation for slit width
1x10^-3/Number of slits
Define coherence
Same frequency and phase difference
Define path difference
Difference in path traversed by the 2 waves, measured in terms of wavelength
Coherence in respect to double slit experiment would mean…
Constant phase difference and frequency
Fringes if the slit is too wide means that…
Slit pattern gets narrower - loses contrast
Define refraction
When light travels through a material because it interact with particles in the material, it slows down. The more optically dense a material is, the more it slows down
Define absolute refractive index
Ratio of speed of light in a vacuum (c) in the speed of light in the material (v)
When does refractive index = 1?
When in a vacuum
When does refractive index >1?
All other materials
What happens to λ when entering a less dense medium?
λ decreases
Snell’s Law
n1sinθ1 = n2sinθ2
Define polarised light
Oscillations of the wave are restricted to only 1 plane (horizontal or vertical)
What are the ways of polarising light?
-Using a polarisation filter
-By reflection
What happens is 2 polarising (Polaroid) filters are used perpendicularly to one another?
1st one reduces ware into one plane halves (light intensity)
2nd one reduces the wave completely (reduces other plane)
Define wave-particle duality
Electromagnetic radiation can act as a wave or a particle
What is the photoelectric effect?
Light shone onto metal plate, causing elections to be released metal atoms to produce a measurable current.
Name of electrons released in the photoelectric effect
Photoelectrons
Define photosensitive
Materials that readily release photoelectrons
How can a single atom be ionised?
Electrons on the outer shell can be ionised if a discrete amount of energy is absorbed by the atom
How does a golden leaf electroscope work?
-If negatively charged, the gold leaf is repelled from the zinc rod
-When UV light is shone at it, some electrons gain enough energy to escape the zinc plate, neutralising it & the gold leaf falls
Evidence for the photon model
-For any given metal with radiation below a certain threshold frequency, no elections are released, even is radiation is intense
-Provided frequency is above the threshold, electrons released instantaneously, even is radiation is weak
-More intense radiation = more electrons released
-E[k] of individual photoelectrons depends only on frequency of radiation
If E[ph] < Φ, then…
Electron is unable to do sufficient work & cannot escape
If E[ph] = Φ, then…
Electrons can be released, but cannot move away from the atom
If E[ph] > Φ, then…
Electron can escape with a kinetic energy (E[ph] - Φ = E[k])
When is the electronvolt (eV) used?
When dealing with individual electrons moving through a p.d. of a few volts
1eV = …J?
1.6x10^-19 (charge of an electron)
How to measure threshold frequency
-Electron’s E[k] can be measured with a photocell & opposing p.d.
- No opposing voltage = flow of current
-Opposing voltage forces elections to move to negative terminal & lose E[k] equivalent to the work done on it by the cell
-Pico ammeter then reads 0
-p.d. that just stops the flow = stopping potential
What is the gradient & y-intercept of eV = hf - hf0?
Gradient = Planck’s constant
y-intercept = Φ
What did Einstein suggest EM waves could exist as?
Discrete packets of energy (photons)
Photon properties
-Can act as particles
-Will either transfer all or none of its energy when interacting with another particle
-Have no charge (neutral)
Define electron energy levels
Energy values that electrons could have are limited to a small number of exact values in free atoms (e.g. gas)
Define ground state
Electron in ‘normal’ circumstances. Lowest energy level, with quantum number of n=1
How can an electron be lifted from the ground state?
By absorbing a photon with the exact same energy - lifts electron up & photon no longer exist
What happens when an electron is already excited?
It will de-excite after a random amount of time
What happens when an electron is de-excited?
-Drops either to ground state or an intermediate level (if there is one)
- Since electron now has less energy, a photon is emitted with the difference in energy
Define ionisation
-n= ∞ has energy value 0, electron has left atom here
-This means that the energy required to ionise an atom in its ground state, its ionisation energy is equal to energy at the ground state (n=1)
What does an absorption spectra look like?
-Coloured background
-Black lines
What does an emission spectra look like?
-Black background
-Coloured lines
Define photon
Discrete packet of electromagnetic radiation
Define threshold frequency
Minimum frequency for there to be enough energy for electrons to be emitted from the surface
Define Φ (work function)
The minimum amount of energy of a photon needed to release an electron
