Waves And The Particle Nature Of Light Flashcards
Transverse waves
A type of wave in which the particles oscillate at right angles to the direction the wave travels
Longitudinal waves
A type of wave in which the particles oscillate parallel to the wave direction.
Wavelength definition
The distance between two matching points on neighbouring waves, Metres
Amplitude definition
The maximum displacement a point moves from the centre of oscillation (equilibrium) Metres
What is the period of a wave?
The time taken for a point or a wave to move through one complete oscillation, seconds
Frequency equation with period
f = 1/T
What is the frequency of a wave?
The number of oscillations per second, measured in Hz
What is the wave equation?
v = f x λ
Name all the parts of the EM spectrum
- Radio
- Microwaves
- IR
- Visible light
- UV
- X - Ray
- Gamma
Radio waves wavelength
Km - 1m
Micro waves wavelength
10^-2 m
IR wavelength
10^-5 m
Visible light wavelength
10^-7 m
UV wavelength
10^-8 m
X ray wavelength
10^-10 m
Gamma wavelength
10^-12 m
What does an EM wave consist of?
An electric field oscillating perpendicular to a magnetic wave
What are the two types of wave?
Mechanical and electromagnetic
What are mechanical waves?
Waves that physically move particles, such as water waves or sound waves
Which type of waves require a medium?
Mechanical waves
What is meant by two points being in phase?
When the points on a progressive wave are one wavelength apart
What is meant by two points being in antiphase?
When two points are half a wavelength apart on a progressive wave. ie, doing the opposite thing (opposite amplitude etc)
What is diffraction?
What happens when a wave goes around or through a gap, causing it to change directions
What happens when the gap is much bigger than the wavelength?
Little to none diffraction occurs
What happens when the gap is the same width as the wavelength?
Maximum diffraction occurs
What happens when the gap is smaller than the wavelength?
The wave does not transit through the gap
What is Huygens principle?
Wavefronts can be considered as a line of point sources of secondary wavelets.
Huygens principle with diffraction
When a wave transmits through a gap, there are no longer adjacent waves to superpose at the edges, so no destructive inference. Therefore the wavelets are free to propagate, hence the change in direction
What happens when waves meet in phase.
Constructive interference. A bigger amplitude is produced, the sum of the two waves amplitudes.
What happens when waves meet in antiphase?
Destructive interference. The waves cancel each other out.
What are maxima?
Areas were waves meet in phase ans create maximum disturbance.
What are minima?
Areas where waves meet in anti phase and create minimum disturbance.
What is a wavefront?
The line in which all the molecules are oscillating in phase
Superposition
When two waves amplitudes add together
What is the phase of a point on a wave?
The position in oscillation
What does it mean when two waves are coherent?
When the waves have a constant phase difference: the same wavelength, frequency, and velocity.
What is path difference
The difference in distance from source to receiver. Measured in wavelength or metres.
What is Young’s slit experiment?
- The first real proof that light travels as a wave
- As the light passes through each slit, the light is diffracted.
- The two diffracted beams superpose eachother, in some place they meet in phase and some in antiphase
- Where they meet in phase, constructive interference occurs and a maxima is produced
- Where they meet in antiphase, destructive interference occurs and a minima is produced.
What happens when the path difference of two waves that meet is an integer?
The waves meet in phase, constructive interference occurs and a maxima is produced
What happens when the path difference of two waves that meet is a non interger?
The waves meet in antiphase, destructive interference occurs and a minima is produced.
What is the equation for wavelength with distance between slits, distance between slits and receiver, distance between maxima?
λ = dw/D where;
λ = wavelength
d = distance between slits
w = distance between maxima
D = distance between slits and receiver
What is the Young double slit equation?
nλ = dSinθ Where;
n = the nth order of maxima
λ = wavelength
d = distance between slits, m
What does the nth order of maxima refer to?
The number of maxima from the central maxima in an interference pattern
Sources of radio waves
Oscillations in electrical circuits
Lightning, stars, nebulas
Uses of radio waves
Communication, radio telescopes, tv broadcasting
Dangers of radio waves
Very little; some heating can occur of biological tissue from high exposures.
Sources of micro waves
Currents in electrical circuits, CMB, stars, other astronomical objects
Uses of micro waves
Heating food, communications, satellites
Dangers of microwaves
Can cause burns due to internal heating of tissue
Sources of infrared
Solar radiation, fire, heating devices, IR remotes
Uses of infrared
Electrical heaters, ovens, remote controls, thermal cameras, fibre optics
Dangers of infrared
burns, damage to tissue
Sources of visible light
The sun, bulbs, LED, lasers
Uses of visible light
Illumination, being able to see, photography, fibre optics
Dangers of visible light
Can cause damage to retina in high exposures
Sources of ultraviolet
The sun, fluorescent lamps, excitation and de-excitation of electrons
Uses of ultraviolet
Disinfect surfaces, reduces pollutants in water and air, can help treat cancer
Dangers of ultraviolet
Sunburn, skin cancer, eye damage
Sources of x-rays
Radon gas, cosmic rays, x-ray machines, radioactive decay
Uses of X-rays
Diagnosis of medical conditions, x-ray telescopes, scanning luggage in airports
Dangers of X-rays
Cancer due to inhiation of DNA, skin burns
Sources of Gamma rays
astronomical objects such as stars, pulsars, supernovas, blackholes. nuclear explosions, radioactive decay
Uses of gamma rays
Kills cancer cells, kills bacteria, used as a tracer, gamma telescopes
Dangers of gamma rays
Ionising radiation can damage tissue and DNA leading to cancer, burns
What is the principle of superposition?
Where two or more waves meet, the total displacement at any point is the sum of the displacements that each individual wave would cause at that point
How does a single slit create an interference pattern?
- We can consider the 2 edges of the slit to be 2 Huygens type sources
- The light from each source interferes
- As they have constant phase differences they are coherent
- And so a regular diffraction pattern occurs
What are the key features of single slit interference patterns?
- The central maximum is twice as wide as the other fridges
- The central maximin is much brighter than the other fridges
- Peak intensity decreases with distance
- Distance between fringes is constant
How is constructive interference defined as path difference?
nλ.
I.e. 0, 1, 2 λ
How is destructive interference defined as path difference?
(n + 1/2) λ.
I.e. 1.5, 2.5, 0.5 λ
What is a node?
A point along a standing wave of zero amplitude
What is an antinode?
A point along a standing wave with maximum amplitude
Difference between progressive and standing waves
1) Each point along a progressive wave has equal amplitude, but for standing waves the amplitude varies
2) Adjacent point on a progressive wave vibrate with different phase but all particles between nodes in standing waves vibrate in phase
3) Energy is transferred through space in a progressive wave but not in the case of standing waves
Equation for velocity of a standing wave in a string
V = √T / μ
Where;
- V = velocity
- T = tension
- μ = mass per unit length
Equation that relates the fundamental frequency to the frequency of the n harmonic
fn = nf1 (fundamental)
How do you draw a standing wave on a string?
Antinodes at each end
How do you draw a standing wave in a closed pipe?
Node at closed end, antinode at open end
How do you draw a standing wave in an open pipe?
Node at each end
What is Kundt’s tube?
A tube with a loose material, such as powder, is closed off at each end, with one end being a loudspeaker connected to a signal generator.
Because the tube is closed, the standing wave acts how it would along a string. Therefore, the wavelength of the first harmonic can be calculate by doubling the length of the tube.
f = v / λ can be used to find the frequency that harmonics would occur at.
What is the photo electric effect?
When light above a threshold frequency is shone on charged metal, the metal will become discharged as the light supplies enough energy for the electrons to overcome the electro static forces and leave the metal.
What are the two variations of the equation for energy in a photon?
E= hf and E = h x c/λ
Where:
E = energy / joules
H = plancks constant (6.63 x10^-34)
f = frequency
c = speed of light
λ = wavelength
What is the work function of a metal?
The minimum energy required for an electron to leave a metal
φ
What is einsteins photoelectric equation
hf = φ + Ekmax
What does the negative y intercept on a graphical representation of Einstein’s photoelectric equation represent?
The work function
What does the x intercept on a graphical representation of Einstein’s photoelectric equation represent?
The threshold frequency of the light
What does the gradient on a graphical representation of Einstein’s photoelectric equation represent?
Planck’s constant
What is Planck’s constant?
6.63x10^-34 Js
Define electron volt
1eV is the energy transferred when an electron moved through a P.D of 1 volt.
What does 1eV equal in joules?
1.6x10^-19 J
Describe how photons are produced
- Electrons are given kinetic energy by a voltage
- The electrons collide with atoms
- If the energy is enough, electrons in atoms will excite to upper shells
- When they de excite, the electrons release energy in the form of light (photons)
What is ionisation?
The removal of an electron from its atomic shell, resulting in a delocalised electron and a positive atomic ion
What is the ionisation energy?
The energy lost by the incident particle; the energy required for ionisation.
What happens when less than the ionisation energy is transferred to an electron?
It excites into a higher energy state
What is excitation using photons?
When an electron is excited by photons
When will excitation using photons occur?
When the photon has energy exactly equal to the difference in energy of the initial and final energy level
What is de excitation?
When electrons return to a lower energy state after excitation.
They release the energy gained in the form of photons.
Equation for emitted proton energy
Energy = hf = E2 - E1
E2 being the higher energy state and E1 being the lower energy state
Equation for refraction index between two materials
1n2 = sin i1 / sin r1 = v1 / v2 = λ1 / λ2
Absolute refractive index definition.
The refractive index for light travelling from a vacuum into the material
What is the equation for absolute refractive index? (Snell’s law)
n1 x Sinθ1 = n2 x Sinθ2
Where;
- n1 = absolute refractive index of first material
- θ1 = angle of incidence
- n2 = absolute refractive index of second material
- θ2 = angle of refraction
What is the equation linking absolute refractive index and the speed of light?
n = c / v
Where;
n = absolute refractive index of the material
c = speed of light in a vacuum (3x10^8 ms^-1)
v = speed of light in material
Why is the potential electro static energy of an electron zero at ionisation?
- Work is done against electrostatic forces when an electron moves up energy levels
- This is stored as potential energy in the electron
- The further up energy levels you go, the more work needs to be done. Potential energy increases with distance.
- But, when the electron is far away enough, the electrostatic force is negligible.
- Potential energy is negative at ionisation, and lower energy levels have an increasingly negative value
What is the equation for wavelength of a photo emitted through de-excitation
λ = (hc) / (E2 - E1)
Where;
h = planck’s constant
c = speed of photon
E2 = energy of higher energy state
E1 = energy of lower energy state
What are emission spectra?
The wavelength that photons which are being emitted by excitation are at
What are absorption spectra?
Where only certain wavelengths of photons are being absorbed by atoms, as they require certain amounts of energy, hence certain wavelengths, to excite electrons
What happens when sound waves enter a denser medium?
They speed up, and refract away from the normal
What is wave particle duality?
The idea that all particles not only have properties of a particle but also of a wave
What is De Broglie’s equation
λdB = h/mv
Where;
- λdB = the De Broglie wavelength
- h = plancks constant
-m = mass of particle
- v = velocity of particle
Describe an experiment to prove that particles can act as waves
- Free delocalised electrons are accelerated through a large P.D, of around 5kv.
- They pass through a thin layer of graphene, causing the electrons to diffract through the graphene atoms
- They hit a phosphorous sheet which reveals an interference pattern
- This interference proves that electrons can act as waves
What happens when voltage is lowered when creating a diffraction pattern with electrons?
- The electrons will diffract more, as KE is proportional to p.d, KE is proportional to v, v is indirectly proportional to wavelength
- The intensity will be lower as less incident electrons are emitted.
How did De Broglie reach his equation?
By deriving it from Einstein’s mass and speed of light equation, E = mc^2, and Planck’s equation, E = hf. He also replaced c with v, as particles cannot move at the speed of light.
Explain what the Bohr Radius is
The most probable distance from the nucleus of a hydrogen atom to an electron in its ground state.
What is the value of the Bohr radius?
5.29x10^-11 m
What does the gradient equal in a graph of n (maxima) against sinθ
d/λ
How is a path difference of one wavelength expressed as a phase difference?
0°, 360°, or 2π
How is a path difference of half a wavelength expressed as a phase difference?
180°, or 1π
