Chapter 3 - Quantum phenomena Flashcards
Briefly describe the photoelectric effect
When photons are absorbed by electrons on metal’s surface and if photon’s frequency is above the threshold frequency, then photoelectrons are emitted
What are the conclusions made from the photoelectric effect?
- No photoelectrons emitted below threshold frequency
- photoelectrons emitted with variety of kinetic energies depending on frequency of photon
- kinetic energy of photoelectrons is unaffected by intensity of photons
- number of photoelectrons emitted per second is proportional to intensity of photons
What does the photoelectric effect give evidence for?
Particle properties of light
- wave theory doesn’t explain why intensity of photon does’t increase kinetic energy of photoelectrons released, but particle theory does.
- Energy increases with intensity in waves
Calculate the wavelength of a photon with energy 3.06x10^-19J?
E =hf
E = hc/wavelength
wavelength = hc/E
= 6.63x10^-34 x 3.0x10^8/3.06x10^-19
= 6.5x10^-7m
Describe an experiment used to prove the photoelectric effect
zinc plate attached to top of an electroscope (box containing piece of metal with a strip of gold leaf attached). Zinc plate is negatively charged - repelling the gold leaf. UV light is shone onto plate - causing electrons to be lost via photoelectric effect. Zinc plate therefore loses negative charge and gold leaf is no longer repelled and so falls back down
What is the work function of a metal?
Energy an electron in a metal needs to break the bonds holding it there, so it can leave the surface.
Minimum energy of photon that causes electron emission from metal surface
Write the equation for threshold frequency using knowledge for work function?
threshold frequency = work function/ planck’s constant
For a metal with work function of 7.2 x 10^-19 J, the minimum frequency of EM radiation needed for photoelectron to be released would be?
threshold frequency = work function/ planck’s constant
= 7.2x10^-19/ 6.63x10^-34
= 1.09 x 10^15 Hz
What is the equation of kinetic energy for a photoelectron leaving metals surface?
Ek max = hf - work function
How do you work out velocity of photoelectron released?
Vmax^2 = 2Ek max / mass of electron
What is an electronvolt?
Kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt
1eV = 1.6x10^-19J
What is excitation?
Movement of an electron to a higher energy level. Happens when an electron absorbs a photon
What happens when electrons move down energy levels?
Releases a photon (equal to energy between 2 energy levels)
Give equation for transition between 2 energy levels
change in energy = energy of initial energy level - energy of final energy level = photon energy (hf)
What is ionisation?
When an electron is removed from an atom, forming an ion
How does a fluorescent tube work?
Contain mercury vapour (conductive at low pressure). When a high voltage is applied - fast moving electrons are accelerated and by electron bombardment mercury atom’s electrons are excited . Mercury electrons are excited to a higher energy level - the electrons then de-excite and release high-energy photons (mainly UV) in stages- each have different wavelengths. The phosphur coating an inside of tube then absorbs these photons - exciting it’s electrons and then releasing photons of various wavelengths (form visible light ) as it cascades down the energy levels - when de-exciting
What is the line emission spectrum and how does it work?
- Seen as bright lines against a black background, each line corresponding to a particular wavelength of light emitted by a source.
- works by diffracting light of different wavelengths at different angles
What do line spectrums show?
Evidence that electrons in atoms exist in discrete energy levels as atoms can only emmit photons with energies equal to difference between energy levels
What is line absorption spectrum?
When white light is passed through a cool gas. Photons of correct wavelengths are then absorbed by electrons to excite them to higher energy levels. Wavelengths are then missing from continuous spectrum of outside the gas - these are the absorbed wavelengths.
What shows that light acts asa particle and what shows that it acts as a wave?
particle - photoelectric effect
wave - diffraction
What is the de broglie equation?
de broglie wavelength = planck’s constant/ momentum
What does the de broglie equation do?
- relates wave property (wavelength) to particle property (momentum)
- Showing particles can show wave like properties
Describe how electrons where shown to have wave like properties?
electrons accelerated to high velocities in a vacuum and then passed through a graphite crystal. They diffract just like waves as they pass through spaces between atoms of crystal. They produce pattern of rings seen in electron diffraction tube
Why does spread of lines in diffraction pattern increase as the velocity decreases
using de broglie’s equation, as the velocity decreases, momentum decreases, so the wavelength increases - meaning diffraction spacing increases.
What effect does increasing frequency of light have on emitted electrons?
Increases energy of light, and therefore increases kinetic energy of photo electrons emitted
What effect does increasing the intensity of the light have on emitted electrons?
More photons are striking the surface so amount of photoelectrons emitted increases
Why might a photoelectron released not have maximum kinetic energy?
Photon and electron have a 1 on 1 interaction. Electron below the surface needs to do work to reach the surface
How does photoelectric effect support the particle nature of EM waves?
light travels as photons. photons have energy that depends on frequency; if frequency below the threshold frequency, no emission even if intensity increased because the energy of the photon is less than the work function. Wave theory can not explain this as energy of wave increases with intensity. Lack of time delay and 1 on 1 interaction between photon and electron could not be explained by wave model
At what energy is the ionisation level (gives ionisation energy) on an energy level diagram
0 eV/ 0J
At the top of the energy level diagram
What happens when an atom in the ground stateis given more energy than it’s ionisation energy?
Electron in the ground state leave the atom with remianing energy as kinetic. Electrons in the higher energy levels fall to fill the vacancy in the lower levels - emitting photons
What is ground state?
When electrons are in their lowest/minimum energy state/ most stable state
Why is it easier to prove the wave-like nature of electrons, than protons?
Easier to accelerate electrons than protons/ easier to get wavelength the same as scattering object
Explain how emission of light from the fluorescent screen shows that electrons incident on it are behaving as particles?
Electrons must provide enough kinetic energy to cause excitation - energy transfer in a 1 to 1 interaction (energy not provided over time like it would be in a wave). Electrons collide with atoms in fluorescent screen causing it’s electrons to excite, which then de-excite - releasing photons as they drop back down to the lower energy level - emmitting light.
