Section 2 - Electromagnetic Radiation and Quantum Phenomena Flashcards
Are electromagnetic waves transverse or longitudinal?
Transverse
What phenomenon can be used to show that light behaves as a particle?
The photoelectric effect
Describe the photoelectric effect
When light above the threshold frequency is shone on a metal surface, electrons are released - these released electrons are photoelectrons
What is the threshold frequency?
The minimum frequency of light required for an electron to be emitted
What equation is used to determine the energy of a photon?
E = hf = hc/λ
Why does a photon need to have a minimum frequency in order to liberate an electron?
- The energy of the photon is determined by its frequency
- The photon’s energy must be greater than the work function in order for an electron to be emitted
If a photon has a frequency higher than the threshold frequency, what would occur?
The electron will be liberated and the remaining energy is the kinetic energy of the electron
If light is incident on a metal and photoelectric emission does NOT occur what is the effect of increasing light intensity?
- If it is more intense then there would be more photons incident on the metal each second
- However each photon still carries the same amount of energy as before
- Therefore it still does not contain enough energy to liberate an electron
- No effect
What is the definition of the work function?
The energy required by an electron to overcome the metallic bond holding it in the metal
What is the photoelectric equation?
hf = ϕ + Ek(max)
What is the definition of stopping potential?
The p.d. needed to stop the fastest moving electrons, with Ek (max)
What is the equation for stopping potential?
e Vs = Ek (max)
where e - charge on the electron
Vs - stopping potential
Ek (max)
What is an electron volt?
The kinetic energy of an electron that has been accelerated from rest through a potential difference of 1V
How do you convert electron volts to joules (and vice versa)?
Electron volt - Joules x1.6x10^-19
Joules - Electron volts /1.6x10^-19
How does a fluorescent tube work?
- High voltage applied across a mercury vapour which accelerates fast moving free electrons which collide with the mercury atoms
- Mercury electrons are excited and then return to the ground state, releasing a UV photon
- The tube’s phosphorus coating absorbs the UV photons and its electrons are excited, they cascade down the energy levels and emit visible light photons
What can be used as evidence for the discrete energy levels in atoms?
- Line emission and absorption spectra - the lines appear at discrete points
- This shows where a light photon of specific frequency and wavelength has been absorbed or emitted
- This then shows electrons can only absorb an exact amount of energy to be excited to the next discrete energy level
What is wave particle duality?
All particles have both particle and wave properties, waves can have particle properties
How and why does the amount of diffraction change when the momentum is changed?
- If you increase the electron speed, and therefore the electron momentum, the wavelength is shorter, and the spread of lines is smaller
- If particles with a greater mass were travelling at the same speed as the electrons, the momentum would be greater, the wavelength shorter, and the spread of lines is smaller
What had to be done before de Broglie could have his theory accepted?
- Evaluation by peer review
- Tested experimentally
- Theory was then accepted and validated by the scientific community
