particles and waves (part 2) Flashcards
equation for energy of a photon
E=hf
what is wave-particle duality?
light being observed as a wave or particle, depending on how we observe it
how can electrons escape an atom?
by absorbing the minimum energy required, Eo (work function), and minimum frequency, fo (threshold frequency), from a photon of light
how can we allow for more electrons to be released?
increasing irradiance of light source, as long as its above threshold frequency
what is the work function?
the minimum energy required to release an electron from a surface
what is the threshold frequency?
the minimum frequency of the photon of light that allows the electron to be released from an atom
work function equation
Eo=hfo
what is condition is required for the electron to have kinetic energy?
energy of photon must be greater than the work function- this extra energy would appear as kinetic energy
equation for kinetic energy of an electron
Ek= E-Eo
what is the photoelectric effect?
when light of a specific frequency falls on a metal plate and ejects electrons from its surface
what evidence is there for the particle nature of light?
each photon removes one electron from surface
what evidence is there for light being a wave?
phenomenon of interference of light
when is interference observed?
when two waves with the same frequency and wavelength overlap
what is the effect of interference dependant on?
whether the waves are in phase or out of phase
what is constructive interference and when does it occur?
when two waves of equal amplitude meet in phase and combine to give a wave of twice the amplitude
when a wave crests coincide or wave troughs coincide
3 things coherent waves share
same frequency, wavelength, and phase
what is destructive interference and when does it occur?
when two waves of equal amplitude meet out of phase and combine to give a wave of zero amplitude
when a wave crest coincides with a wave trough
what part of a water wave displays constructive interference?
bold lines moving outward from ripple
what part of a water wave displays destructive interference?
faded gaps in between each section of lines
example of the interference of light
youngs double slit experiment
monochromatic light shone through single slit producing a source of coherent waves
pass through a double slit to produce two coherent sources of waves
interference fringes produced on screen consisting of bands of bright light (constructive interference) and dark bands (destructive interference)
what happens as frequency increases?
wavelength decreases
maxima path difference equation
pd= nλ
minima path difference equation
pd= (n+1/2)λ
how does a grating work?
made up of many slits placed very close together
light passes through and depending on path difference to screen, a particular interference pattern is produced
equation for wavelength of laser and fringe seperation
mλ= dsinθ
equation for lines between slits
d= 1/lines per mm
notation for nm
10^-9
why is central maximum always white?
path difference to it is zero for all wavelengths or colours of light
these superimpose on each other to produce white light
what colour is always closest to central maxima
violet
what is the refractive index?
the refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium
(constant produced when plotting the sine of the angle of incidence against the sine of the angle of refraction)
refractive index formula
n= sinθ1/sinθ2
equation for snells law (full refractive index equation)
n2sinθ2 = n1sinθ1
what does the refractive index measure?
the effect a medium has on light
the greater the refractive index, the…
greater the change in speed and direction
equating snells law, velocity, and wavelength
n2/n1 = sinθ1/sinθ2 = v1/v2 = λ1/λ2
equating velocity and wavelength
v1/λ1 = v2/λ2
what is irradiance?
the power per unit area
(i.e. watts per square metres)
irradiance formula
I= P/A
irradiance units
wm^-2
how to show the relationship between irradiance and distance
light meter measures light levels at a range of distances from the light bulb in an otherwise completely dark room
what does a graph of irradiance against the inverse of squared distance prove?
irradiance is directly proportional to the inverse of the squared distance
levels of irradiance with changing distance from source formula
i1d1² = i2d2²
(i = k/d² )
(D(i1d1² = k))
when can an electron move to a higher energy level?
if the electron gains energy but only at discrete energies
difference between energy levels formula
E2-E1= hf
what happens when an electron loses energy and moves to a lower level?
emits a photon
what is seen in the line spectrum for sodium light?
two yellow lines that are close together, which are especially bright
due to there being a lot of electrons making that energy jump, not because it is a large energy jump
how can electrons move up energy levels?
absorb photons of light that contain the required amount of energy and a particular frequency since E= hf
what happens when many electrons are making the same upward jump of energy levels?
a reduction in the number of photons with that energy level and of a particular frequency
why are there missing lines in the emission spectrum of the sun?
gases present in the outer part of the sun absorb photons of particular frequencies
absorption lines produced as a result correspond to the emission lines of particular elements- allowing identification of elements present in gas
what is found when passing white light through an element in gaseous form?
results in dark bands appearing in the spectrum of white light
two dark lines found where the bright yellow line emission spectrum are found
