Quantum physics Flashcards
Photon
A quantum of energy in the form of electromagnetic radiation
Formula for energy
hf
Momentum of a photon formula
P= E/c
where c is the speed of light
When a charged particle is accelerated in an electric field, its v is calculated how
by equating eV= 1/2 mv²
Photoelectric effect?
When electromagnetic radiation of threshold frequency falls on a metal surface, electrons are emitted
Threshold frequency
Minimum frequency required to release electrons from the surface of a metal
Threshold wavelength
Maximum wavelength required to release electrons from the surface of a metal
Work function
Minimum amount of energy required by an electron to escape from its metal surface
Work function formula
∅ = hfo
= Plank’s constant * threshold frequency
Rules of photoelectric effect (3)
- A single photon can release a single electron
- Energy is conserved during the interaction
- If energy is sufficient, surface electrons are released instantaneously
Einstein’s photoelectric equation
Energy of a photon= Work function+ Max. Ke
hf = hfo + ½ m(vmax)2
Stopping potential
P.d which stops electrons with max. ke from reaching the anode in a photocell
Factors affecting the ke of photoelectrons are (2)
- frequency of incident photons
- work function of the metal surface
Photoelectric emission
is the release of electrons from surface of the metal when e.m radiation is incident to the surface
Intensity formula in terms of photons
Rate of arrival of electrons * energy of each photon
Why do electrons have a high range of KE values (2)
- If the frequency is greater than threshold, surface electrons will escape and have surplus energy in the form of KE- KEmax
- If the photon interacts with electrons below the surface, some energy is used in bringing the electron to the surface so is emitted with less KE
Evidence for particle theory
Photoelectric effect as light wave energy is transferred in the form of photos and interacts with other matter such as electrons
Light is a wave evidence
Light is propagated through space as a wave.
Evidence is in interference and diffraction of waves
Three suggestions of wave theory of light
- any frequency of light can give rise to photoelectric emission if exposure time is long enough
- Energy absorbed by each electron will increase gradually with each wave
- KE of emitted electrons will increase with radiation
Wave theory suggests;
any frequency can release electrons
This is wrong because:
Photoelectrons will be released immediately if frequency is above threshold frequency of the metal
Wave theory suggests;
Energy absorbed by electrons increases
This is wrong because:
Energy is absorbed instantaneously-
photoelectrons are either emitted or not
emitted after exposure to light.
Wave theory suggests;
KE will increase with intensity
This is wrong because:
If the intensity of light is increased more
photoelectrons are emitted per second.
When does electron diffraction occur? (2)
- electrons are accelerated to high speeds
- pass through a gap similar to their size
De broglie wavelength formula
= h/mv
Emission line spectra (2)
- composed of light emitted from hot gases
- dark background with coloured lines
Absorption line spectra (2)
- observed when white light is passed through cool gases
- coloured background with dark lines
Ground state
condition where the atom possesses the minimum possible energy
Excited state
atoms absorb energy so the energy of the atom incerases
When electrons move from higher to lower energy level, what happens
they emit energy in the from of photon of me radiation
For electrons to move from lower to higher energy level, what needs to happen
Absorption of a photon
Use the concept of discrete energy levels to
explain the existence of dark lines (5)
in the absorption spectra of white light passing through cool gas
- photons give energy to electron
- electron moves to higher energy level
- energy is equal to difference in energy levels
- electron de-excites giving off photon of same energy
- photons emitted in all directions
Pieces of evidence provided by photoelectric effecet for particulate nature of electromagnetic radiation
5
- max. energy of emitted electrons depends on frequency not intensity
- rate of emission of electrons depends on intensity
- existence of frequency below which no emission of electrons
- instantaneous emission of electrons
- increasing frequency at constant intensity decreases rate of emission of electrons
what is de broglie wavelength
wavelength associated with a moving particle
