Unit 2.7: Photons Flashcards
Other than behaving as waves, how else can light behave?
As a stream of particles
Energy in a light wave is not a continuous stream but rather is quantized into what?
Little bundles of energy known as QUANTA or PHOTONS
What did concept did Max Planck introduce?
That energy of a photon is a fixed amount
Concept of quantisation of energy
What did Einstein conclude from the quantisation of energy?
That the energy of a photon is proportional to the frequency of radiation
What is a photon?
A discrete packet of energy
What is the equation for the energy of a photon?
Ephot=hf or E=hc/λ
What is one electron volt(eV)?
One electronvolt is the energy of an electron accelerated by a potential difference of 1 Volt
What unit is used to describe the energy of a single photon?
Joule
What is 1eV in Joules
1eV=1.6x10^-19J
What is the typical wavelength for Gamma waves?
10^-12m
What is the typical wavelength for X-ray waves?
10^-10m
What is the typical wavelength for UV waves?
10^-7 or 10^-8
What is the typical wavelength for visible waves?
4x10^-7m to 7x10^-7m
4=Blue
7=Red
What is the typical wavelength for infrared waves?
10^-5m
What is the typical wavelength for Microwaves?
10^-2
What is the typical wavelength for radio waves?
10^2
What is the typical photon energy for Gamma waves?
10^6 eV
What is the typical photon energy for an X-ray wave?
10^4 eV
What is the typical photon energy for UV?
10^1 eV
What is the typical photon energy for visible light?
3.1eV to 1.8eV
blue red
What is the typical photon energy for infrared waves?
10^-1eV
What is the typical photon energy for microwaves?
10^-4eV
What is the typical photon energy for radio waves?
10^-8eV
What is the photoelectric effect?
When light or UV radiation of short enough wavelength falls on a surface, electrons are emitted from the surface
What is work function(Φ)?
The work function is the minimum energy required to remove an electrons from the surface
What are the units for Work function?
Joules or electronvolts
What is threshold frequency (f0)?
The minimum frequency of a photon that will cause the emission of an electron from a particular metal surface
What happens when EM radiation of enough energy strikes the cathode surface?
Electrons are ejected
What are photoelectrons?
Ejected electrons from the surface of a cathode
During the photoelectric effect experiment, where do the photoelectrons travel and what does this create in the circuit?
The photoelectrons travel across to the anode and a current flows in the circuit
During the photoelectric effect experiment, how is current read?
Using an ammeter
During the photoelectric effect experiment, the p.d is increased in the circuit it until what happens?
The photoelectrons are being repelled from the anode and at certain p.d the electrons won’t have enough energy to reach the anode
The cathode and anode have to be in what during the photoelectric effect experiment
They must be in a vacuum otherwise the ejected electrons could collide with molecules in the air
State the steps for the photoelectric experiment
- Shine the light on cathode
- Adjust p.d until micro-ammeter shows 0 current
- Read voltmeter
- Plot a graph, current against p.d
If the intensity of light increases, what happens to the number of photons of light
They increase
Ek max=Evstop
State Einstein’s photoelectric equation
Ekmax=hf-Φ
If the energy of a photon is just enough to cause an electron to be ejected then te photoelectron will have no Ek. State the photoelectric equation with no Ek.
hf0=Φ
Ekmax=hf0-Φ & y=mx+c predicts what on a photoelectric graph
That Ekmax against f will be a straight line
How do you find threshold frequency on a photoelectric graph?
Where the line crosses the x axis
What idea did Niels Bohr develop in 1913
-Energy in atoms was quantised and the the electrons existed in specific energy levels
According to Niels Bohr, when can electrons move between energy levels?
When they are given the correct quantity of energy usually in the form of a photon
If an electron want to move up an energy level, it must absorb what particle?
Photon
If an electrons wants to move down an energy level is must emit what particle?
Photon
What is ionisation energy?
The minimum energy required to remove an electron from the atom in its ground state
How does an absorption spectra form in terms of electrons?
When light passes through clouds of gas in the star’s atmosphere and certain wavelengths are absorbed as electrons are promoted to higher energy levels
What is the emission spectrum?
Electrons collide with atoms in the gas and cause electrons to be promoted to higher energy levels. We get coloured lines.
What is electron diffraction?
When a beam of electrons is accelerated through a thin polycrystalline graphite film in an evacuated chamber with a fluorescent screen to see where the electrons come out the other side. This creates a diffraction grating
