Waves Flashcards
What is refraction
Refraction is the change in velocity of light when it passes from one medium to another.
If light is incident on a boundary at an angle to the normal other than 90 degrees it will also change direction.
When light travels from a less dense to a more dense material the ray moves […]
towards the normal
When light travels from a more dense to a less dense material the ray moves […]
away from the normal.
What is absolute refraction index (refractive index)
The absolute refraction index, n, of a material is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v).
What is the refractive index of a vacuum and of air?
1
For any media other than air or a vacuum what is true about the refractive index, n?
N is always greater than 1 and has no units (no units for all media)
As refractive index increases what happens?
the change in speed and direction of light as it passes from air into the medium increases.
The denser a material the greater the…
refractive index
does refractive index change depending on if the light moves from air into the medium or the medium into air?
No, the refractive index is the same whether the light moves from air into the medium or the medium into air.
State Snell’s law
Snell’s law states:
(n2/n1) = (sin01/sin02)
where n1 and n2 are the refractive indices of the two media.
When one medium is air what happens to the snells law relation ship
Since air has a refractive index of 1 the relationship becomes:
(n2/1) = (sin01/sin02)
or as given on the formula sheet:
n = sin01/sin02
where n = refractive index of medium
sin01 angle in air
sin02 angle in medium
What does refractive index of a medium depend on
frequency of light, refractive index increases with the frequency of light.
Why does a prism split white light into its spectrum of colour
because the prism medium has a different refractive index for each colour of light.
Why does a prism refract violet light more than red light?
As violet has a higher frequency than red light, the refractive index for the prism medium is greater for violet than for red light, so violet light refracts more.
Does frequency of light change
Frequency of light does not change.
What is the relationship between wavelength, speed of light and angle in refraction
The wavelength and speed of light changes with the same ratio as the angles when light is refracted by a medium.
What is the critical angle
The critical angle is the angle of incidence in a medium when the angle of refraction in air is 90 degrees.
This is only relevant when light travels from a higher refractive index medium to a lower refractive index medium.
What happens if the angle of refraction is increased past 90 degrees
No light is refracted and total internal reflection takes place.
When angle in the material is less than the critical angle what happens
Most light is refracted, there is a weak partially reflected ray.
When angle in the material is equal to the critcal angle what happens
Light is refracted at 90 degrees, there is a stronger reflected ray.
When angle in the material is bigger than the critical angle what happens
No light refracts. All light is reflected. Total internal reflection occurs.
What is the formula for critical angle
Sin0c = 1/n
where n is refractive index.
What 3 things can all waves do
reflect, refract, diffract.
What is the relationship between diffraction and wavelength
Longer wavelengths diffract more than shorter wavelengths.
What is interference?
When two waves meet they combine to produce a new pattern - this is called interference.
What is interference evidence for
Interference is evidence for the wave model of light.
What two ways can waves interfere in
Constructive interference and destructive interference.
What 3 things does pattern of interference depend on
The pattern depends on the wave direction, amplitude and wavelength.
What must be true to observe interference patterns
To observe interference patterns the waves must come from coherent sources.
What makes waves coherent
Coherent waves must have a constant phase relationship (but are not necessarily in phase)
When does constructive interference occur
Constructive interference occurs when waves meet in phase (crest meets crest/trough meets trough)
What does constructive interference cause
Constructive interference results in an increase in amplitude.
When does destructive interference occur
Destructive interference occurs when waves meet out of phase (creast meets trough)
What does destructive interference cause
Destructive interference results in a reduced (or zero) amplitude.
When do interference patterns occur
when two coherent sources meet.
How can two coherent sources be made
by making two sets of waves from a single coherent monochromatic (single frequency) light source directed towards a double slit.
When does a maximum (or “bright fringe”) occur
when the waves meet in phase and constructive interference has taken place.
When does a minimum (or “dark fringe”) occur
when the waves meet out of phase and destructive interference has taken place.
What must happen for a maximum to be made
one wave must have travelled whole number of wavelengths further than the other.
What is the formula for path difference of a maximum
path difference for a maximum = mλ
where m is the order number of the maxima and λ is wavelength.
What is the path difference at the central maximum
at the central maximum (zero order) both waves have travelled the same distance so the path difference is 0
What must happen for a minimum to be made
For a minimum to be made, one wave must have travelled an odd number of half wavelengths further than the other.
What is the formula for path difference for a minimum
path difference for a minimum = (m+1/2)λ
NOTE: first minimum occurs when the path difference is λ/2 so m = 0 at first minima
What factors effect fringe seperation
Fringe seperation can be increased by:
increasing wavelength
Decreasing slit seperation
Increasing the distance between the slits and the screen.
Whaat does a diffraction grating do
A diffraction grating makes an interference pattern brighter and more spaced out. It lets a lot more light through than a double slit. It contains many equally spaced lines very close together.
What is the diffraction grating equation
dsin0 = mλ
where:
d = the line of seperation of the grating (metres)
0 = angle from the zero order to the mth order (degrees)
λ = wavelength (metres)
m = order of the maximum
How do you find the spacing between lines in a grating (in metres)
d = 1mm/number of lines
(Milimeters on top unless stated otherwise)
How can you increase the angle of diffraction (and therefore the distance between fringes)
Increase wavelength of the lightsources
Decrease the slit seperation (d) (more lines per mm)
moving the screen further away causes increase in fringe seperation but not angle.
Why is the central maximum white
All colours constructively interfere.
What is the approximate wavelength of red light
650nm
What is the approximate wavelength of green light
540nm
What is the approximate wavelength of blue light
490nm
Highest order maxima are …
Spectra (have coloured edges)
What is true about the angle at which maxima occur
It is bigger for longer wavelengths, therefore the maxima for red light appear at larger angles than the maxima for blue light.
What are the 3 key features of the white light interference pattern (using a grating)
The central fringe is white as for all colours of light the path difference is 0 (pd = (0*λ))
Violet light has a shorter wavelength than red light so violet light pattern is less spread out than for red light.
Many spectra are produced, symmetrical about the central maximum.
What happens when white light is shone through a prism
The light is split into the visible spectrum
What are the 4 key features of the white light refraction pattern
Only one spectrum produced by refraction.
Violet deviated the most, red the least as violet has a higher refractive index for mediums compared to red light.
Bright image compared to a grating spectrum that is less intense as energy is divided between several spectra.
Less spread out than a grating spectrum.
Describe the bohr model of the atom
It states that the orbits of electrons are quantised. This means that they can only exist at certain distances from the nucleus. Electrons exist only in discrete energy levels.
Is the energy between each level the same
The energy between each energy level is not the same.
What is the lowest energy level called
The ground state
What does the term excited state mean
If an electron has been moved from its normal level to a higher energy it is said to be in an excited state.
How can you move an electron from the ground state
It is possible to move an electron from the ground state to an excited state by inputting energy. This is often done in the form of light.
What causes emmision lines on a spectrum
Each emmision line on a spectrum occurs when an electron makes a transition between an excited state, E2, an a lower state, E1.
What happens to excess energy when an electorn moves from an excited state to a lower stated
The excess energy is emmited as a photon of light. The frequency of the radiation emmited depends on the energy of the photon. The energy of the photon depends on the nergy levels the electron jumps between.
What is the formula for energy of photons.
hf = E2-E1
where:
h = plancks constant (6.63*10^-34Js)
f = frequency (Hz)
E2 and E1 = energy levels in an atom (J)
What are the 3 types of spectra
Continnous spectrum
Line emmision spectra
Line absorption spectra
What produces a continuous spectrum
Produced by an object or gases which radiate heat.
What produces a line emmision spectra
Produced by electrons making transition from excited states to a lower energy level releasing a photon of a particular frequency.
What produces a line absorption spectra
Produced when photons of light pass through a gas. The photons with the same energy as the energy gaps in the atoms can be absorbed.
Define light irradiance on a surface
Light irradiance on a surface is defined as the power on every metre squared.
State the irradiance formula
I = P/A
where:
I = Irradiance (Wm^-2)
P = power (W)
A = area (m^2)
What is the relationship between light irradiance and distance from a point source
Light irradiance is inversely proportional to the square of the distance from a point source, or:
I1d1^2 = I2d2^2
Where:
I1 = irradiance at point 1 (Wm^-2)
I2 = irradiance at point 2 (Wm^-2)
d1 = distance to point 1 (m)
d2= distance to point 2 (m)
Define the photoelectric effect
The ejection of electrons from a metal surface by photons of sufficient energy on the surface.
The photoelectric effect is evidence for…
the particle model of light.
Light can be thought of as a stream of particles called…
Photons
A photon can be thought of as…
a bundle of energy
The phenomenon of light having both particle and wave properties is called…
wave-particle duality
Explain how electrons are ejected from the surface of a metal
Electrons on the surface of a metal are held on by an attractive force which must be overcome for the electrons to escape the metal surface, this is done by increasing the electrons energy.
What is work function
The minimum energy given to each electrons to allow it to escape from the surface.
How do you find energy of photons
The energy of photons is directly proportional to the frequency
E =hf
E is energy of the photon
h is plancks constant
f is frequency of photons
If one photon strikes a metal surface, it can cause…
One electron to be emmited from the metal surface if the photon has an energy equal to or greater than the work function
Define threshold frequency
threshold frequency is the minimum frequency required for a photon to have the minimum energy (work function) required to emit an electron.
Photons below the threshold frequency will…
not emmit electrons from the metals surface
Any photons equal to or greater then the threshold frequency will…
emit photoelectrons from the metals surface.
What is photoelectric current
When electrons are emmited by an incident photon, there is moving charge (a current) which is known as the photoelectric current.
Graph of photoelectric current against frequency will show what relationship
as the frequency increases above the threshold frequecny the photoelectric current will increase.
State the relationship for kinetic energy of an ejected electron
Ek = hf - hfo
Where hfo is the workfunction
hf is the energy of the incident photon
and ek is the kinetic energy of the ejected electron
This can be rewriten as:
1/2 m v^2 = hf - hfo
If the frequency of photons is less than the threshold frequency
No electrons are emmited
If the frequency of photons is greater than the threshold frequency
Electrons are emmited with some kinetic energy
If the frequency of photons is far greater than the threshold frequency
Electrons are emmited with lots of kinetic energy.
Providing the frequency of the light is above the threshold frequency then increasing the irradiance will
increase the number of electrons ejected
Increasing the irradiance when the frequency of the incident light is below the freshold frequency will
will not eject electrons.
