4.5 Quantum Flashcards
What is a photon?
A quantum of light of electromagnetic radiation.
The energy of each photon depends on only one property. What is it?
Frequency.
True or false: a photon has no charge and no mass.
True.
What does the h in E = hf represent, and what are its units?
The h represents Planck’s constant, which has units Js. Its value is given in the formula book.
The electronvolt (eV) is a unit of what?
Energy.
Why is the electronvolt used as a unit of energy for photons?
The energy of a photon is very small when measured in joules
How is 1 electronvolt defined?
1 electronvolt is defined as the energy transferred when an electron travels through a potential difference of 1 volt.
How can you determine the number of joules in an electronvolt?
Using the formula W=VQ, the work done is equal to the charge on an electron multiplied by the p.d., 1 volt, so an electronvolt = 1.60×10⁻¹⁹ joules.
What is the photoelectric effect?
When EM radiation (light) falls on a metal surface, electrons are emitted so
long as the frequency of the EM radiation is above a certain ‘threshold’
value. The emitted electrons, called photoelectrons, have kinetic energies
ranging from zero to a maximum value.
Which features of the photoelectric effect can’t be explained if light is a wave?
If light was a wave, then the energy of the electrons released would increase with increasing intensity of the light - but this isn’t the case. Instead, the energy of the electrons depends on frequency (and no electrons are released below a certain threshold value, no matter how intense the light is).
How many photons does each photoelectron absorb prior to emission?
Only 1. If it doesn’t contain enough energy the electron will re-emit the energy rather than being released.
How does the photon model of light explain the threshold frequency seen in the photoelectric effect?
Each electron absorbs a single photon. This single photon must have enough energy for the electron to be released, if it doesn’t the energy is re-emitted. The electron can’t build up energy as it could if light was a wave.
What is the name given to the minimum amount of energy an electron requires to leave the surface of a metal?
The work function (or ‘work function energy’), 𝜙.
True or false: the maximum kinetic energy of photoelectrons is independent of the intensity of the incident radiation.
True.
What experimental evidence appears to show particles behaving as waves?
Electron diffraction.
- Electrons will diffract is passed through the spaces between atoms in graphite (like a tiny diffraction grating).
- This wouldn’t happen if electrons were behaving as particles only.
