Photoelectric Effect Flashcards
(36 cards)
Define the photoelectric effect
Where photoelectrons are emitted from the surface of a metal when a light above a certain frequency is shone on it
Define the threshold frequency
The minimum frequency of light needed for photoelectrons to be emitted from the surface of the metal
Why can’t threshold frequency be explained by wave theory
Wave theory suggests that any frequency of light could cause photoelectrons to be emitted as energy absorbed by each electrons would gradually increase with each wave
How can threshold frequency be explained by the photon model of light
- EM waves travel in discrete photon packets, whose energy is directly proportional to frequency
- each electrons absorbs a single photon, so can only be emitted if frequency > threshold frequency
- if light intensity is increased and frequency is above the threshold, more photoelectrons are emitted per second
How does light intensity affect photoelectron emission
More photoelectrons are emitted if intensity increases
Define the work function of a metal
The minimum energy required for photoelectrons to be emitted from the surface of the metal
Define the stopping potential
The potential difference needed to stop the photoelectrons with the maximum kinetic energy
What equation can be used to find stopping potential
Ek max = e Vs
- e is the charge of an electron
- Vs is the stopping potential
What is the photoelectric equation
E = h f = work function + Ek max
How to electrons in atoms exist
In discrete energy levels
How do electrons in discrete energy levels gain energy
By colliding with free electrons
When does ionisation occur, with reference to energy
When the energy of the free electron is greater than the ionisation energy
Define excitation
When electrons gain energy so move up in energy level
What happens when an electrons de-excites
It releases the energy is gained in the form of a photon
Give a use of excitation
In fluorescent tubes to produce light
What’s inside a fluorescent tube
Mercury vapour
Explain how a fluorescent use works
1) high voltage is applied across tube
2) voltage accelerates free electrons through tube
3) free electrons collide with mercury atoms, ionising them so releasing more free electrons
4) free electrons collide with mercury atoms, exciting them
5) mercury atoms release UV photons when they de-excite
6) the phosphorus fluorescent coating absorbs UV photons
7) electrons in atoms of coating excite
8) electrons in atoms of coating de-excite, releasing photons of visible light
Define an electron volt
The energy gained by one electrons when passing through a potential difference of 1 volt
How do you produce a line spectrum
Pass light emitted from a fluorescent tube through a diffraction grating
What does passing the light from a fluorescent tube through a diffraction grating produce
A line spectrum
What does each line in a line spectrum represent
A different wavelength of light emitted by the tube
Why is the line spectrum not continuous
It contains discrete values of wavelength
What would be produced is you passed white light through a cooled gas
A line absorption spectrum
