Photons and the Photoelectric Effect

what is a photon?

• a discrete packet of energy

• photons are what electromagnetic radiation is made up of when it interacts with matter

what is the equation for the energy of a photon?

• E=hf

• where E= energy of the photon, f=frequency of the emr, h=the Planck constant, 6.63×10^-34

what is the energy of a photon in terms of its wavelength

E=hc/wavelength

what is an electronvolt?

the energy transferred to or from an electron when it moves through a potential difference of 1V (it is 1.6×10^-19J)

what is the photoelectric effect?

when high energy electromagnetic radiation (which is made up of photons) is incident on a metal, electrons are emitted from the surface of the metal

what is a photoelectron?

an electron which has been emitted from the surface of a metal during the photoelectric effect

what is the gold-leaf electroscope practical?

• in the beginning, the top plate is touched with a negative electrode, charging it with excess electrons

• the negative charge from the plate spreads to the stem and the gold leaf

• the stem and gold leaf both become negatively charged and drift away from each other

• when zinc is placed on the electroscope and uv radiation is shone onto the zinc, the radiation causes photoelectrons to be emitted from the zinc

• as the electroscope loses its negative charge, the gold leaf stops repelling the stem and drifts back towards it

what are the observations from the photoelectric effect?

• photoelectrons are only emitted if incident radiation is above the threshold frequency

• if incident radiation is above the threshold frequency, emission of photoelectrons is instantaneous

• if incident radiation>threshold frequency, increasing intensity does not increase the kinetic energy; it just increases the number of electrons

• to increase kinetic energy, increase frequency of incident radiation

what is the work function?

• the minimum energy required to free an electron from the surface of a metal

• each photon transfers its exact energy to one surface electron in a one-to-one interaction

how does increasing intensity increase the number of photoelectrons?

• more intensity means more photons per second

• as each photon interacts one-on-one with a surface electron, more photons per second means more photoelectrons emitted per second

• rate of emission of photoelectrons is also directly proportional to intensity

how does increasing frequency increase kinetic energy of photoelectrons?

• when frequency increases, energy of photon increases (E=hf)

• when energy of photon increases, it is transferred to the electron, so the electron has more kinetic energy left after being emitted from the surface