4.1 Wave And Quantam Behaviour Flashcards
How do u calculate frequency with an oscilloscope?
- measures voltage over time
- move voltage per division and time per division dials to give waves
- find time period and multiple by time per division
- 1/T= frequency
How do u use standing waves to calculate the speed of sound?
- Hollow tube in into measuring cylinder of water
- get tuning fork and note down frquency
- gently tap so sound wave travels and reflected at water surface where a node is formed
- move tube up and down until u find shortest distance that it resoantes (makes loud sound)
- measure distance (this is 1/4 wavelength)
- find speed from v=f lambda
- repeat w different frequencies
What is refraction?
When a wave changes direction as it enters a different medium
How does refraction occur?
- When a ray enters mew medium some energy is reflected back and rest transferred into new medium
- if it enters at and angle to the normal it changes speed
- frequency is constant so also changes wavelength
If a ray of light travels into a more optically dense medium what happens.
- if medium is denser then lught interacts w more particles so slows down more as more energy is transferred
- the ray bends toward the normal
- the wavelength decreases
What is absolute refractive index and how is it calculated?
-Ratio of speed of light in a vaccum to speed in that medium so is
n=c/cmedium
How do u calculate refractive index?
Sini / Sinr=C1/ C2=n2/n1
What can be said about airs refractive index?
- Speed of light very similar to C so assume n is 1
- sini / sinr=n (of medium)
What is diffraction?
When waves spread out as they come through a small gap
How to show diffraction with a ripple tank?
- change gap size
- most diffraction when gap is equal to wavelength
Showing diffraction with an obstacle
- when a wave meets an obstacle it diffracts round the edges
- forms triangular type of shadow behind obstacle
- when length of obstacle gets larger than wave length the wave diffracts less so gives longer shadow
Explain diffraction of a beam of light through a split?
- if wavelength of light wave is similar to the size of apeture split) u get light and dark fringes on a screen
- bright central fringe w dark and brigth alternation fringes on either side decreasion in intensity as less photons incident per unit aread
Why is the brightest point the centre in a single split experiment?
Light travels in straight line so all waves are in phase
How to bright fringes appear in single split experiment?
-infinite sources of wavelets can interfere with each other
-Brightest fringe are in phase so point straight
For other bright points:
-constant phase difference
-so waves super impose but phasors are in slightly different directions
-form smaller resultant
Why do dark fringes appear in single split?
- phase difference between waves give a resultant phasor of zero
- phasor forms a loop
What has to be true about waves making a diffraction pattenr?
Need to be coherent ( same frequency and wavelength and comstant phase differentce)
How is double split set up?
- when a laser is shined at a double slit it can act as two coherent point sources
- the slits must be similar sized to wavelngth so light can be difffracted
- a pattern of light and dark fringes appear depnding on constructive or destructive interferenc
What is youngs double slit formula
X (fringe spacing) =D (distance from screen) x wavelength / d (split spacing)
What is youngs double slit evidence for and what does it show?
- wave nature of light
- shows diffraction and interference
What dies a diffraction grating do for monochromatic loght?
- lots of equally spaces slits
- gives brighter and narrower fringes and darker dark fringes
- because so many beams reinforce interfecenrce pattern
- gives more accurate measurements
What are zero, first and second order lines in light passed thrpigh diffraction grating?
- maximum centre kine is zero order line
- next bright fringe out is first order and son on
How do you derive youngs double slit from diffraction grating equation?
- small angle approximation for first order tan feta= feta and sinfeta=feta
- make a tan triangle
- show X/D =tan feta = feta= sin feta (bc small angle)
- sub in gives you Xd/D= wave length x 1 (n=1)
- rearrangel for X
How is a spectra produced from white light?
White light is a mixture of colours with different wavelengths
- through a diffraction grating the patterns due to different wavelmgths are spread out by different ammounts
- each order except the zero order become a spectra withe red on outside and violet on inside
- the spectras help identify elements
What is a photon
-discrete packets of energy called quanta
What is an electron volt?
-kinteic energy gained by an electron when it is accelerated through a pd of 1 vold
How are LEDs used to estimate Plancks constant?
- LED of known wavelength in circuit placed in very dark room
- slowly adjust power source until a cirrent just begins to flow and LEDlights up
- record V ( threshhold voltage) across LED
- repeat w different coloured LEDs of different wavelngths
- plot V against frequency to get straight line graph of gradient h/e
- multiply by e for h
- repeat for average
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What does photoelectric effect show?
-particle behaviour of light
Explain photo electric effect
- when light hits its surface, metal is bombarded w photons
- if one of these photons is absorbed by a free electron the electron gains E=hf energy
- if its enough energy to break bonds holding it there its is emmited
How does increase in light intensity affect photo elctrons?
- each electron only absorbs one photon at a time so all energy for a release comes from one photon NOT a build up
- so im reased light intensity (more photons) wont affect ke of electrons only frequency will
Photo electric effect conclusions
- For a given metal no photoelectrons emitted if radiation is below threshhold frequency
- value of KE increases w frequency and uneffected by light intensity
What is number of photo electrons emmited per second proportional to?
Intnesity
Why cant photo electric effect be explaimed by wave theory?
- according to wave theory even if light had a low frequency it would give a little bit of energy for eac hwave and build up over time
- so the higher intensity the more energy so ke would increase w intensity but it doesnt
Explain work function
-work function of a metal is energy needed to break bonds holding it there
-if energy gained is greater than work finction of metal electron is emitted
-if notmetal heats up
hf>work function
Equation for work function
F=work function/h
Explain energy levels
- electrons exist in energy levels
- it can move down by emitting a photon
- energy carried by photon is equal to difference in energy levels
How is a line emission spectra produced?
- gas heated to high temp, excites atoms so moves electrons to higher energy levels
- as they fall back to ground state certain photons emitted
- split light thorugh a diffraction grating gives a line spectra
- each line correspomds to particular wavelength of light emitted from the photons of soecific energies
How is an absorbtion spectra formed.
- white light passes through a cool gas most electrons are in ground state
- photons of correct wavelengths are absorbed by electrons to excited them to higher energy levels
- these wavelengths are then missing from spectrum
- can work out elements in gas by which wavelengths are absorbed
What can be said about photon paths?
-Photons take all possible paths at omce
How to use phasors to find probability of arrival
-most paths cancel so consider straightest/quickest paths
-add phasors at the end of every path to give resultant phasor
-(longer paths will have rotated further as same frequency)
-probability is proportional to the resultant phasor squared
-
Which path gives highest probaiblity
-quickest route as will contribute most to resultant amplitude
Summary of electron diffraction
-electrons accelerated in a vaccum passed through graphite crystal and interact with the spaces to form rings of bright and dark fringes
-use phasors to find how likely it is, higher probability brighter point on screen
-evidnece for debrogile wavelngth as 1/2mv^2=hv/lambda
A higher voltage give higher Velocity gives smaller wavelength and smaller line spread
Which fits with lambda=h/mv
Explain debrogile wavelength
- says that particles display quantam bahvaioir
- increasing accelerating volatage increases speed so shorter wavelengths so smaller spread of lines
How dpes E=hf differ for electrons
E is the kinetic energy
When do electrons not show qunatam behaviour?
- only diffract w objects of similar wavelengths
- shorter wavelngth diffracts less
What is a standing wave, how do they form?
Standing waves form when two identical waves travelling in opposite directions superimpose forming nodes and anti nodes.
Usually a wave is reflected to form a standing wave
What do bright and dark fringes show about probability of an electron arriving at a point?
Bright fringes show high probability as many electrons have arrived here and interference pattern has built up to make bright fringes
How are we able to identify when an electron has hit the screen?
When electron hits screen photon is released
