Photons Flashcards
What is a photon?
A discrete packet of energy.
What is the equation for energy of a photon?
E=hf or E=hc/λ
What is the electron volt (eV)?
The energy of an electron accelerated by a potential difference of one volt.
What does one eV equal in joules?
1eV= 1.6x10^-19J
What is a gamma rays typical wavelength and photon energy?
Wavelength: 10^-12m
Photon energy: 10^6eV
What is a x-rays typical wavelength and photon energy?
Wavelength: 10^-10m
Photon energy: 10^4eV
What is a ultraviolet waves typical wavelength and photon energy?
Wavelength: 10^-7 or 10^-8m
Photon energy: 10^1eV
What is a visible waves typical wavelength and photon energy?
Wavelength: 4x10^-7 to 7x10^-7 m
Photon energy: 3.1 to 1.8eV
What is an infrared waves typical wavelength and photon energy?
Wavelength: 10^-5m
Photon energy: 10^-1eV
What is a microwaves typical wavelength and photon energy?
Wavelength: 10^-2m
Photon energy: 10^-4eV
What is a radio waves typical wavelength and photon energy?
Wavelength: 10^2m
Photon energy: 10^-8eV
What is the photoelectric effect?
When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface.
What is the work function?
The minimum energy needed to remove an electron from the surface.
What is the threshold frequency?
The minimum frequency of a photon that will cause the emission of an electron from a particular metal surface.
Describe the photoelectric effect experiment
When EM radiation of enough energy strikes the cathode surface, electrons are ejected. The photoelectrons travel across to the anode which creates current, read by the ammeter. The potential difference is increased until the photoelectrons are being repelled from the anode. At a certain pd electrons wont have enough energy to reach the anode.
Why must the cathode and anode be in a vacuum in the photoelectric effect experiment?
To avoid ejected electrons colliding with air molecules.
Describe the steps in carrying out the photoelectric effect experiment
- Shine light on cathode
- Increase pd until micro ammeter reaches zero
- Read voltmeter
- Plot a graph of current over pd
What is Einsteins photoelectric equation?
Ekmax= hf-ϕ
What is Ekmax?
The maximum energy a photoelectrons can have once it has used some of the photon energy to leave the metal surface.
What is hf?
The energy of the photon (J).
How does the equation change when the photoelectrons has no Ek?
hf0=ϕ
What is some evidence for the particle nature of light?
If the light is not of a high enough frequency, then it does not matter for how long it is shone, photons cannot combine to give the electron sufficient energy.
What is the y-intercept of a Ekmax over frequency graph?
The metals work function (ϕ).
What is the gradient of a Ekmax over frequency graph?
Planks constant (6.63x10^-34).
What happens if an electron wants to move up an energy level?
It must absorb a photon, with the photon energy being equal to the energy gap.
What happens if an electron wants to move down an energy level?
It must emit a photon, with the photon energy being equal to the energy gap.
What is the ionisation energy?
The minimum energy needed to remove an electron from the atom in its ground state (n=1 to n=∞).
What experiment can be performed to show wave-particle duality?
Accelerating a beam of electrons through a thin polycrystallite graphite film in an evacuated chamber results in an interference pattern of dark and light fringes, these are electron diffraction rings.
What is the deBroglie equation?
λ=h/p or p=hf/c where p=momentum (kgms-1).
What can the deBroglie equation also be used to find?
The wavelength of particles.
