topic 5 Flashcards
amplitude
the maximum magnitude of the displacement
frequency
number of cycles (vibrations) per second passing a given point
period
time taken for whole cycle to complete
wavelength
length of one whole wave cycle, e.g. from crest to crest or trough to trough
longitudinal waves
waves oscillate parallel to direction of energy transfer.
- made up of compressions and rarefractions
pressure variation at rarefraction vs compression stages
pressure: increased at compression and decreased at rarefraction point
displacement of particles at rarefraction vs compression stage
rarefraction: neighbouring particles move away from eachother
compressiom: neighbouring particles move towards a point
transverse waves
waves oscillate perpendicular to direction of energy transfer.
all EM waves are transverse (speed = 3x10^8)
wavefront
surface which is used to represent the points of a wave which have the same phase
coherence
when two light sources have the same frequency and wavelength and fixed phase difference
path difference
difference in distant travelled by two waves
superposition
When two or more waves cross at a point, the displacement at that point is equal to the sum of the displacements of the individual waves
interference
when two or more waves superpose with each other
phase
measurement of the position of a certain point along the wave cycle
in phase
when two waves are at the same point of the wave cycle, hence they have the same frequency and wavelength (coherent) and the phase difference is a multiple of 360° (2π radians)
out of phase
when two waves have the same frequency and wavelength (coherent) and their phase difference is an odd integer multiple of 180° (π radians)
stationary/standing wave
formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude
how standing wave is formed
- in phase: constructive interference occurs so anti-nodes formed (area of max displacement)
- out of phase: destructive interference occurs so nodes formed (regions of no displacement)
intensity
power per unit area
I = P/A
refractive index
- measures how much it slows down light passing through it
n=c/v - a material with a high refractive index can be known as being more optically dense
refraction
when a wave enters a medium and causing it to change direction, either towards or away from the normal which causes the wave to either slow down or speed up
Snell’s law
n1sinθ1 = n2sinθ2
- n1: refractive index of material 1
- n2: refractive index of material 2
- θ1: angle of incidence of the ray in material 1
- θ2: angle of incidence in material 2
critical angle
sinC = 1/n
reflection
the wave bounces back when it hits a boundary
total internal reflection (TIR)
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 boundary (n2)
measuring refractive index
converging lenses
- bring light rays together
- cause parallel light rays to move closer together
- focal length, f, is the distance between lens axis and focal plane
- f is positive for converging lens because it is in front of the lens
diverging lens
- cave inwards
- cause parallel light rays to move apart
- focal length, f, is distance between the lens axis and principal focus
- f is negative for diverging lens because it is behind the lens
real image
formed when light rays from a point on an object pass through another point in space
can be projected onto a screen
virtual image
formed when light rays from a point on an object appear to have come from another point in space
cannot be projected onto a screen
what does converging lens form
converging lens can form both real and virtual images. If the object is further than the focal length, the image is real. If the object is closer, the image is virtual
what image does a diverging lens form
always form a virtual image - the position of object in relation to focal length will not affect the type of image formed
lens equation
1/f = 1/u + 1/v
power
tells you how much a lens bends light. can be calculated by P = 1/f
diffraction
the way that waves spread out as they pass through a narrow gap or go around obstacles
how does diffraction become more noticable
as the gap decreases, the diffraction becomes more noticeable. if the gap becomes too small the water waves cannot pass through it anymore
use Huygen’s construction to explain what happens to a wave when it meets a slit or an obstacle
the secondary wavelets that pass through the slit produce the curve of the new wavefront emerging from the slit
measuring the wavelength of light using a diffraction grating
1) position a laser in front of a diffraction grating so that the light travels through the grating and creates an interference pattern on a flat wall or screen a few metres away
2) measure the distance, D, between the diffraction grating and the wall
3) measure the distance, x, between the zero order maximum and the 1st order maximum for both sides and take an avg of the two readings
4) repeat the measurements for more order lines to find an avg wavelength
wavelength of light
nλ = dsinθ
general conclusions from nλ = dsinθ
1) if λ is bigger, sinθ is bigger, so θ is bigger. This means the larger the wavelength, the more the pattern will spread out
2) if d is bigger, sinθ is smaller. This means that the coarser the grating, the less the pattern will spread out
3) values of sinθ greater than 1 are impossible
plane polarisation
wave only oscillates in one direction
polarising filter
- when filters are aligned all of the light that passes through 1st filter also passes through the 2nd
- rotating the 2nd filter the amount of light that passes through varies
- as 2nd filter rotates, less light will pass
- at 90 degrees no light will pass through
light - wave or photon? why?
- both
- light produces interference and diffraction patterns - alternating bands of dark and light. this can only be explained using waves.
- photoelectric effect is only explainable if light behaved as a particle - known as a photon
what is a photon
a quantum of EM radiation
- Einstein suggested EM waves can only exist in discrete packets - photons
- photons have no charge
equation that relates photon energy to wave frequency
E = hf = hc
λ
how do electrons move down energy levels
by emitting a photon
electronvolt - eV
1.60 x 10^-19 J
how do electrons move up energy levels
by absorbing a photon with the exact energy difference between the two levels
hot gases - atomic line spectra
produce line emission spectra
- atoms become excited when heated so move to higher energy levels
- as electrons within the atom fall back down, they emit photons
- if you split light from a hot gas within a diffraction grating, a line emission spectra is produced (black one with thin coloured lines)
-
cool gases - atomic line spectra
produce line absorption spectra
- at low temps, most electrons in the gas will be in their ground states
- when white light passes through a cool gas and
then a diffraction grating, a line absorption spectra is produced (colour one with thin black lines)
- photons of correct wavelength are absorbed by electrons to excite them to higher energy levels
how does the photoelectric effect provide evidence for the particle nature of EM radiation - photoelectric effect
- free electrons on the surface of a metal absorb energy from light
- if an electron absorbs enough energy the bonds holding it to the metal break and electron is released
first conclusion drawn from the photoelectric effect
- for a given metal no photoelectrons are emitted if the radiation has a frequency below a certain value - threshold frequency
second conclusion drawn from the photoelectric effect
the photoelectrons are emitted with a variety of kinetic energies ranging from zero to a max. value. This value of max KE increases with the frequency of the radiation and is unaffected by the intensity of the radiation
third conclusion drawn from the photoelectric effect
the number of photoelectrons emitted per second is proportional to the intensity of the radiation
threshold frequency
the minimum frequency of incident radiation below which the photoelectric emission is not possible completely
equation linking threshold frequency and work function
hf = φ + ½mv^2
light as a wave - wave model
newton’s theory suggested light could interfere and diffract (both wave like properties)
light as a particle - photon model
after 100 years, Einstein explained that beam of light was a series of particle-like photons
if a photon is a discrete bundle of energy, then it can interact with an electron
all the energy in the photon is given to one electron
De Broglie equation
λ = h / p
how does a pulse-echo technique provide information about the
position of an object
you send out a sound wave to an object an unknown distance awa
using ultrasound waves in scans
- frequency of ultrasound waves is too high for humans to hear
- ultrasound is directed into body via a transducer
- gel is applied to skin so there is no air between transducer and skin
- when the waves reach an interface inside body, some of them are reflected
- computer detects how long it takes for reflected waves to return
why are shorter pulses used
produces clearer images
- pulses of ultrasound transmitted must be very short so reflections from nearby interfaces don’t reach transducer
- gap between pulses must be long so all reflected waves from one pulse return to the transducer before the next pulse is transmitted
resonant frequency
minimum
