Waves And Particle Nature Of Light Spec Points Flashcards
Amplitude
A wave’s maximum displacement from the equilibrium position
Frequency
The number of complete oscillations passing through a point per second
Period
Time taken for one full oscillation
Speed
Distance travelled by the wave per unit time
Wavelength
The length of one whole oscillation (e.g. the distance between successive peaks/troughs)
Longitudinal waves in terms of pressure variation
Pressure is decreased in the rarefactions and increased in the compressions
Longitudinal waves in terms of the displacement of particles
Rarefaction - neighbouring particles move away from each other
Compression - neighbouring particles move towards a point
What is the speed of all EM waves in a vacuum
3e8 ms-1
CP06: speed of sound using 2-beam oscilloscope, signal generator, speaker and microphone
Variables and equipment
IV - distance
DV - phase of received signals
CV - same location to carry out the experiment
- for each set of readings, the same frequency of sound
- signal generator with loudspeaker
- oscilloscope with 2-beam facility
- microphone
- 2 metre rulers
- connecting leads
CP06: speed of sound using 2-beam oscilloscope, signal generator, speaker and microphone
Method
- connect microphone and signal generator to an oscilloscope and set up the signal generator about 50cm from microphone
- set the signal to abt 4kHz
- the oscilloscope should trigger when the microphone detects a sound, adjust the time base so that the signal from the generator and the microphone can be on the screen with about three cycles visible
- adjust the separation so a trough on the upper trace coincides with a peak on the lower trace (this makes judging the point where the waves coincide easier)
- record the distance between the microphone and the signal generator (distance one, d1)
- move the microphone further away, watch the traces on the screen
- when the next trough and peak coincide, record the new distance (d2)
- repeat as many times as possible in the available space
- calculate the mean wavelength of the sound
- using the oscilloscope trace find the frequency of the sound
- reduce the frequency to around 2kHz and repeat
CP06: speed of sound using 2-beam oscilloscope, signal generator, speaker and microphone
Analysis of results
Speed of sound - v = f(lambda)
Frequency found from time base of oscilloscope by using f = 1/T
CP06: speed of sound using 2-beam oscilloscope, signal generator, speaker and microphone
Systematic and random errors
Systematic
- ensure the scale of the time base is accounted for correctly
The scale is likely to be small (e.g. milliseconds) so ensure this is taken into account when calculating frequency
- use the oscilloscope signal trace to find frequency to avoid relying on the dial of the signal generator
Random
- reduce by doing repeat readings and taking an average in measurements
- the time interval is small so make the distance between the microphone and signal generator as large as is practical
CP06: speed of sound using 2-beam oscilloscope, signal generator, speaker and microphone
Safety considerations
- the voltage and current are low, so normal care with electrical equipment is sufficient
- keep sound at a normal listening volume to avoid damage to hearing
Phase
The position of a certain point on a wave cycle. This can be measured in radians, degrees or fractions of a cycle
Phase difference
How much a particle/wave lags behind another particle/wave. This can be measured in radians, degrees or fractions of a cycle
Path difference
The difference in the distance travelled by two waves
Superposition
Where the displacements of two waves are combined as they pass each other, the resultant displacement is the vector sum of each wave’s displacement.
Coherence
A coherent light source has the same frequency and wavelength and a fixed phase difference
Wavefront
A wavefront is a surface which is used to represent the points of a wave which have the same phase
Two types of interference are -
And they occur during -
Constructive, destructive, superposition
Constructive interference
Occurs when two waves are in phase and so their displacement is added
Destructive interferecne
Occurs when the waves are completely out of phases and so their displacements are subtracted
If two waves are in phase …
They are both at the same point of the wave cycle, meaning they have the same frequency and wavelength (are coherent) and their phase difference is an integer multiple of 360degrees
Two waves will be completely out of phase when …
They are coherent and the phase difference is an odd integer multiple of 180 degrees
Path difference equation
Path difference = wavelength / 360 degrees x phase difference
What is a stationary/standing wave
Formed from the superposition of two progressive waves, travelling in opposite directions in the same place with the same frequency, wavelength and amplitude
How much energy is transferred by a stationary wave
None
In phase stationary waves
Constructive interference occurs so anitnodes are formed - regions of maximum displacement
Completely out of phase stationary waves
Destructive interference occurs and nodes are formed which are regions of no displacement
Speed of a transverse wave on a string equation
Speed = the square root of tension in the string over the mass per unit length of the string which is constant
Plane polarisation
A polarised wave oscillates in only one plane, only transverse waves can be polarised
How polarised sunglasses work
Reduce glare by blocking partially polarised light reflected from water and tarmac, as they only allow oscillations in the plane of the filter to pass through, making it easier to see.
Diffraction definition
Spreading out of waves when they pass through or around a gap
Huygens’ construction
Every point on a wavefront is a point source to secondary wavelets, which spread out to form the next wavefront.
Can be used to explain the diffraction of light when it meets an obstacle or passes through a gap
How wavelength affects diffraction
If wavelength is a lot smaller than the size of the slit, the wave barely diffracts
Whereas if the wavelength is closer to that of the slit, it diffracts more
The greatest amount of diffraction occurs when the gap is the same size as the wavelength
What is a diffraction grate
A slide containing many equally spaced slits very close together. When light is passed through a diffraction grating, it forms an interference pattern composed of light and dark fringes
Order lines of diffraction grating
The ray of light passing through the centre of a diffraction grating is called the zero order line
Lines either side of the zero order are the first order lines and so on
Diffraction grating equation
dsin0=n(laambda)
d - distance between the slits in the diffraction grating
Theta - angle to the normal made by the maximum (light fringe)
n - the oder
CP08: wavelength of light from a laser or other light source using a diffraction grating
Variables and equipment
IV - distance between maxima
DV - angle between normal and each order
CV - distance between the slits and the screen, D
- laser wavelength
- slit separation , d
Laser
Single slit
Double slit
Diffraction grating
Metre ruler
Vernier callipers
Retort stand
White screen
Set square
CP08: wavelength of light from a laser or other light source using a diffraction grating
Method
- Place the laser on a retort stand and the diffraction grating in front of it
- Use a set square to ensure the beam passes through the grating at normal incidence and meets the screen perpendicularly
- Set the distance D between the grating and the screen to be 1.0m using a metre ruler
- Darken the room and turn on the laser
- Identify the zero-order maximum
- Measure the distance h to the nearest two first-order maxima using vernier calliper
- Calculate the mean of these two values
- Measure distance h for increasing orders
- Repeat with a diffraction grating with a different number of slits per mm
CP08: wavelength of light from a laser or other light source using a diffraction grating
Analysis of results
n(lambda)=dsin0
distance between slits is equal to d = 1/N where N is the number of slits per metre
Theta:
Tan0=h/D
CP08: wavelength of light from a laser or other light source using a diffraction grating
Systematic and random errors
Systematic
- ensure the use of the set square to avoid parallax error in the measurement of the fringe width
- using a grating with more lines per mm will result in greater values of h which lowers percentage unctertainty
Random
- The fringe spacing can be subjective depending on its intensity on the screen, therefore, take multiple measurements of w and h and find the average
- use a vernier scale to record distances w and h to reduce percentage uncertainty
- increase the grating to screen distance D to increase fringe separation
- conduct the experiment in a darkened room so the fringes are clear
CP08: wavelength of light from a laser or other light source using a diffraction grating
Safety
- lasers should be class 2 and have a maximum output of no more than 1mW
- do not allow laser means to shine into anyone’s eyes
- remove reflective surfaces from the room to ensure no laser light is reflected into anyone’s eyes
Core practical 7: investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire.
Aim + variables
To measure how the frequency of the first harmonic is affected by changing either the length, tension or mass per unit length of string
IV: either length, tension, or mass per unit length
DV: frequency of first harmonic
CV: if length is varied, same masses attached and same string etc…
Core practical 7: investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire.
Equipment list
Signal generator
Vibration generator
Retort stand
G clamp
2 metres of string
Pulley
Wooden bridge
Mass hanger and 100g masses
Metre ruler
Top-pan balance
Core practical 7: investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire.
Method
- Set up apparatus by attaching one end of the string to the vibration generator and pass the other end over the bench pulley and secure to the mass hanger.
- Adjust the position of the bridge so that the length L is measured from the vibration generator to the bridge using a metre ruler.
- Turn on the signal generator to set the string oscillating
- Increase the frequency of the vibration generator until the first harmonic is observed and read the frequency that this occurs at
- Repeat procedure with different lengths of L
- Repeat the frequency readings at least two more times and take the average of these measurements
- Measure the tension in the string using T=mg (m is the mass attached to the string and g is the gravitational field strength on Earth)
- Measure the mass per unit length (mass/length)
Core practical 7: investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire.
Analysis of results
For first harmonic, wavelength = 2L
So speed of stationary wave is
f x 2L
y = mx+c
f = v/2 x 1/L + 0
Plot a graph of the mean values of f against 1/L
Draw line of best fit and calculate gradient
Work out wave speed which is 2x gradient
Verify wavespeed using v = sqrt T/mew
Core practical 7: investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire.
Errors
The sharpness of resonance leads to the biggest problem in deciding when the first harmonic is acheived
The measurements would have a greater resolution i f the length used is as large as possible
Core practical 7: investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire.
Safety considerations
Use a rubber string instead of a metal wire incase it snaps under tenison
If using a metal wire wear goggles to protect the eyes
Stand well away from masses incase they fall on the floor
Place a soft surface under the masses to break their fall
Intensity of radiation equation
I = P/A
Refractive index
Property of a material which measures how much it slows down light passing through it
n = c/v
When can TIR occur
When the angle of incidence is greater than the critical angle
And the incident refractive index (n1) is greater than the refractive index of the material at the boundary (n2)
What is the critical angle
When the angle of refraction is exactly 90 degrees, and the light is refracted along the boundary, the angle of incidence has reached its critical angle.
Measure the refractive index of a solid material
- Place the material in the centre of a piece of paper and draw around it using a pencil.
- Next, put the material block aside and mark a point on the outline of the material, preferably in the centre, and draw a line perpendicular to the outline at this point. This is the normal line. Use a protractor to make sure that the line is at exactly 90 degrees.
- Using a protractor, draw lines leaving the point you have marked at 10 degree intervals from 10-70, where the angle is measured from the normal line to the line you are drawing. These will be the incident rays.
- Put the material block back, making sure that it fits the outline as well as possible.
- Using a ray box, shine a ray of light along the 10 degree line and mark the point at which the light ray leaves the material block.
- Join the point you have just marked down to the point on the normal line, at which the light ray enters the block. Using a protractor, measure the angle between this line and the normal. This is the angle of refraction.
- Repeat the above two steps for all of the incident angles.
- Plot a graph of sine of the incident angles against sine of the refracted angles. Plot a lineof best fit and find the gradient. This is the refractive index
Converging lenses
These are curved outwards on both sides and cause parallel light rays to move closer together/ converge at a point
Diverging lenses
Curved inwards on both sides and cause parallel light rays to move apart/ diverge
Principal focus (F)
In a converging lens: the point at which the light rays which are parallel to the principal axis are focused.
In a diverging lens: the point from which the light rays appear to come from
Focal length (f)
The distance from the centre of the lens to the principle focus
Power (lenses)
The measure of a lens’ ability to bend light. In converging lenses this value is positive and in diverging lenses this value is negative
Ray diagrams - how to draw
Draw two lines from the same point of an object, one which passes through the centre of the lens and is left unrefracted and one which moves parallel to the principal axis and passes through the principal focus.
Ray diagrams - real or virtual
If image is real, two lines will meet. If it is virtual, it will appear where the two lines appear to come from. This can be found by drawing a dashed line backwards from both of the initial lines and finding the point they meet
Power of a lens
Power = 1 / focal length
Ability to bend light
Positive - converging
Negative - diverging
Thin lens definition
Thickness which allows rays of light to refract but not experience dispersion or aberrations
Thin lenses which are used in combination act as … with a power equal to …
A single lens
The sum of the powers of the individual lenses
P = p1 + p2 + p3 + …
What is a real image
One which can be projected onto a screen as light rays reach the image location
What is a virtual image
An image that cannot be projected onto a screen
Equation for thin converging/diverging lens to find power
1/u + 1/v = 1/f = power
u - distance between the object and the lens axis
v - distance between the lens axis and the image
f - focal length
Magnification of lens equation
v/u
Ratio of the size of the image it creates with respect to the size of the object
What can electron diffraction experiments be performed using and what does it do
Electron gun
It accelerates electrons through a vacuum tube towards a crystal lattice, where they interact with the small gaps between atoms and form an interference pattern on a fluorescent screen behind the crystal.
How does an electron gun provide evidence for the wave nature of electrons
The interference pattern created looks like a set of concentric rings.
If electrons only had a particle nature, you would expect the pattern to look like a single point, where the electron beam has passed through the lattice. However, this is not the case as the electrons undergo diffraction, which is something only waves can experience. This is why electron diffraction provides evidence for the wave nature of electrons.
de Broglie relation equation
lambda = h / p
lambda - de Broglie wavelength
h - Planck constant
p - momentum of the particle
What is an interface
A boundary between two materials
Transmitted meaning
Where waves pass into the next material. They can experience refraction if the materials have different refractive indices
Reflected meaning
Where the waves bounce off the interface without passing into the next material
Pulse echo technique
- Short pulse ultrasound waves are transmitted into the target
- The pulse travels inside the body until it reaches a boundary between two mediums where some of the pulse is reflected back. The amount of reflection depends on the difference in densities of the materials; the greater this difference, the greater the reflection.
- The reflected waves are detected as they leave the target.
- The intensities of the reflected waves are used to determine the structure of the target and the time taken for these reflected waves to return is used to determine the position of objects in the target (using s=vt)
Pulse echo:
Ways in which the amount of info you retain (the resolution of the image) will decrease
If duration of pulses are too long they will likely overlap
As wavelength increases, less fine details can be resolved
What does the photon model state
EM waves travel in discrete packets called photons, which have an energy directly proportional to their frequency
Define wave model (EM waves)
EM radiation can be described as a transverse wave
What was light initially believes to be
Composed of tiny particles as this could explain the reflection and refraction of light
What was light later proved to be
Act as a wave through diffraction experiments
What is light now believed to be
Due to the discovery of photoelectricity
Both a particle and a wave which led to the development of the photon model of light and wave-particle duality.
Equation for proton energy
E = hf
E - photon energy
h - plancks constant
f - wave frequency
What is the photoelectric effect
Where photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it.
What is threshold frequency
Certain frequency for different types of metals
The minimum frequency of light required to emit photoelectrons
Why are photoelectrons emitted from the surface of a metal after light is shone on it
Because electrons near the surface of the metal absorb a photon and gain enough energy to leave the surface
What is the work function of a metal
The minimum energy required for electrons to be emitted from the surface of a metal, it is denoted by ϕ
Photoelectric equation
E = hf = ϕ+ Ek(max)
E - photon energy
ϕ - work function
Ek(max) - maximum kinetic energy
What is the unit of energy usually used to express small energies
Electronvolt
eV
What is 1eV equal to
The kinetic energy of an electron accelerated across a potential difference of 1V
Or 1.6E-19 Joules
How to convert between joules and electron volts
Joules to electron volts
Divide by 1.6E-19
Elctron volts to joules
Multiply by 1.6E-19
Reasons why the photoelectric effect as evidence for the particle nature of EM radiation couldn’t be explained by wave theory
- wave theory suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave, and so can’t explain the existence of threshold frequency.
- photoelectric effect is immediate, contradicts wave theory which suggests time is needed for the energy supplied to the electrons to reach the work function
- increasing the intensity of the light does not increase the speed of photoelectric emission as would be suggested by wave theory, but instead increases the number of photoelectrons released per second.
- photoelectrons are released with a range of kinetic energies
What does the photon model of EM radiation suggest
That EM waves are released in discrete packets called photons which have particle-like interations
When a photon interacts with an electron
__________ of its energy is transferred to it. An electron interacts with a _______ photon
All
Single
The threshold frequency is the frequency at which the photon energy is equal to …
The work function of the metal
Why are photoelectrons emitted immediately
Photon energy is transferred immediately when the photon interacts with an electron
What is intensity equal to
The number of photons released per second
All electrons will receive ___ _____ ____ of enerfy from a photon of light
The same amount
Electrons in atoms can only exist in ….
Discrete energy levels
What is excitation
When electrons gain enough energy they can move up in energy level
Why will excited electrons quickly return to their original energy level
Because they release the energy it gained in the form of a photon of light
How to get an atomic line spectrum
Passing light from a fluorescent tube through a diffraction grating
Inside the tube, electrons are accelerated, causing gas atoms to become excited and the de-excite, releasing photons
What do the lines in the line spectrum represent
A different wavelength of light emitted by the tube.
As this spectrum is not continuous but rather contains only discrete values of wavelength, the photon energies emitted will correspond to these wavelengths. This is evidence to show that the electrons in atoms can only transition between discrete energy levels
Photon freq equation
f = (E1 - E2) / h
E1/E2 represent energy levels
h is planck constant
