Chapter 17- Particle Nature Of Light Flashcards
What is newton’s corpuscular theory?
He thought of light as being made up of corpuscles (or tiny particles)
He imagined the particles to bounce of the surfaces explaining reflection
What was Huygen’s wave theory?
Light consisted of waves like ripples spreading out like a stone dropped in water
Difference between Huygen’s or newton’s theories?
The differences in our if water or glass and then in air which medium means light has the fastest speed
Intensity def
The power per unit area
Has units W m^-2
Another way of saying intensity
Radiation flux density
Intensity eq
Intensity = power/area
If we are a distance r from point source of radiation, what will happen to the radiation?
It will spread out in all directions over a sphere of surface area 4pie r^2
The photoelectric effect can be demonstrated by what means?
A negatively charged zinc plate and a UV lamp
What happens in photoelectric effect?
The freshly cleaned zinc plate is given a negative charge
Under visible light photoelectrons are discharged at a very low rate
But under slight UV light they are discharged immediately
If zinc plate is positively charged what happens with the photoelectric effect?
Nothing at all
As any photoelectrons transmitted would be attracted to the plate
What is the photoelectric effect?
When energy from the light is given to the electrons causing their emission from the substance
If ONE photon had enough energy then…
It will release ONE photoelectron
What is a quanta now called
A photon
What is a quanta formed from
Discrete bundles of energy
Instead of energy from the light being gradually absorbed by the electrons near the surface….
Einstein reasoned that
The electron would be emitted only if a single quantum of the light had enough energy for the electron to escape
What is Einsteins theory called?
Quantum theory
Why can UV discharge a photoelectron and visible light can’t?
Because UV has a higher frequency than visible light
The intensity of a light source depends on… (2)
The number of quanta
The energy associated with each quantum
Electron volt def
The work done on (or gained by) an electron when it moves through a potential difference of 1 volt
Electron volt unit
eV
1 electron volt has how many joules of energy?
1.6 x10 ^-19 J
What is the bare minimum energy needed to liberate an electron from the surface called?
The work function
Symbol= o with line through it
If a photon of energy hf has more than the work function what does it go into
Kinetic energy
maximum kinetic energy an electron can have eq
hf= 🚫+ 1/2 mvmax^2
What is Einsteins photoelectric eq
hf= 🚫+ 1/2mvmax^2
Electrons are emitted with a range of kinetic energies up to the maximum because…
Because electrons inside the metal will need more than the work function to escape
There will be no photoelectric emission unless…
hf>🚫
Threshold frequency
Frequency just large enough to liberate electrons fo
So 🚫=hfo
What is a phototube?
Name given to a type of photocell that generates photoelectrons when light falls on a specifically coated metal cathode
What is a photovoltaic photocell?
In which an emf is generated by the presence of light across the boundary of two semiconducting materials, the photoconductive cells, or LDRs
Ground state def
The state an electron is in when it is at its lowest possible energy level
Ionisation energy def
The energy that must be supplied for an electron in the lowest energy level to just escape from the atom
Quantum DEFINITION
A fixed or discrete amount of energy
Emission spectrum def
Of a chemical element or compound, is the spectrum of frequencies of electromagnetic radiation emitted when the electrons in an atom make a transition from a high energy state to a lower-energy state
When an electron drops from energy level E2 to a level E1 eq
hf= E2 - E1
Applications of quantum emission (2)
- radar (radio detection and ranging)
* laser
What happens in an absorption spectrum?
Radiation is absorbed by a substance at frequencies that match the energy difference between two quantum states of electrons in an atom or molecule of the substance
What is wave-particle duality?
The concept describing the fact that light behaves like waves and also like a particle
What is Broglie’s wave-particle duality eq?
Wavelength= h/p (momentum)
What is the complementarity principle?
Says that sometimes electrons have the properties of particles and sometimes the properties of waves, but never both together
What is plank’s constant
6.63 x10 ^-34
Energy eq
E= hf
Kinetic energy max eq
KE= hf- 🚫o
Stopping potential def
Point of frequency at which no flow is created as all photoelectrons released are then attracted back to the original cathode as they run out of energy
Stopping potential eq
Vs x charge = E
Therefore..
qVs= hf- 🚫o = KE max
Charge of an electron
1.6 x10 ^-19
Alternative eq to
KE= hf- 🚫
KE= hc/wavelength - 🚫
Electric fields cause
Voltages
A photon can be emitted in
Any direction
Fewer photons therefore come back towards you (intensity decreased)
You can tell the type of elements something is made up from by…
The absorption spectrum
What type of emission when dropping down energy states
Spontaneous emission
Mass of an electron
9.1x10 ^-31
