Section 2 - EM Radiation and Quantum Phenomina Flashcards
What can shining light on a metal do?
Release electrons
How can you release electrons from the surface of a metal?
By making sure the light is a high enough frequency (Normally in UV range)
State the process for electrons to be released from the surface of a metal
- Free electrons of the surface of the metal absorb energy from the light
- If enough energy is absorbed, the bonds holding it to the metal break, and the electron is released
- Called photoelectric effect, electrons emitted are called photoelectrons
What is the name of the electrons emitted in the photoelectric effect?
Photoelectrons
What are the 3 conclusions from the photoelectric effect (brief)
- There is a threshold frequency
- There are a range of kinetic energies, varying on frequency and not intensity
- No. of photoelectrons emitted/second is proportional to intensity
What is the range of kinetic energies for the photoelectrons being emitted?
0 to some maximum value
Why can’t wave theory explain photoelectric effect?
Wave theory predicted that:
- Energy carried is proportional to intensity
- Energy carried would be spread evenly across wavefront
- Each free electron would gain a bit of energy
- Slowly, each electron would gain enough energy to escape
What problems does wave theory have the photoelectric effect?
- No explanation for threshold frequency, as wave theory states that regardless of frequency electrons should be emitted
- KE should increase with intensity, but it does not
- Intensity should increase energy transfer to each electron, but it increase no. emitted/second
State Einstein’s photon model of light
- EM waves exist in packets, called photons
- Energy carried by one of these photons is E=hf
- Photons have one-to-one interaction, like particles with an electron and the metal surface. A photon would transfer all energy to one electron
- Before an electron can escape, it needs enough energy to break bonds holding it there - called work function
What does the photon model explain which wave theory cannot?
The threshold frequency
State how the photon model can explain the threshold frequency
- If E gained by electron from a photon > work function, electron is emitted
- If it isn’t, no electrons are emitted, but metal heats up
- For electrons to be released, hf>= work function, so hf = work function
How does the photon model explain the maximum kinetic energy of a photoelectron
- Energy transferred to electron = hf
- KE electron carries when leaving metal = hf - energy lost on way out. Electrons deeper in the metal lose more energy than ones on surface which explains range
- Minimum energy it can lose is work function
- Max KE of photoelectron is given by hf = work function + Ekmax, where Ekmax = 1/2mv^2
- KE of electrons is independent fo intensity due to one-to-one interaction. Increasing intensity leads to more photons emitted/second
What is the use of the stopping potential?
Gives the max KE
Explain how you can use the stopping potential to find out the maximum KE
- Emitted electrons are losing energy by doing work against an applied PD
- Stopping potential, Vs is the PD needed to stop fastest electrons
- WD by PD stopping fastest electrons = energy they were carrying -> eVs = EKmax
Explain eVs = Ekmax
e = charge of electron Vs = stopping potential Ekmax = max KE
How do electrons exist in atoms?
In discrete energy levels
What is the level of the ground state?
n=1
How can electrons move down an energy level?
By emitting a photon
Why can the energy of a photon only take certain values?
Because the transitions for photons to be emitted are between definite energy levels
Define an electron volt
KE carried by an electron after it has been accelerated through a PD of 1V
What is the energy carried by a photon equal to?
hf, or difference in energies between 2 levels
How can electrons move up an energy level?
If they absorb a photon with the exact energy difference between the 2 levels
What is the movement of electrons to a higher energy level called?
Excitation
What is it called when an electron is removed from an atom?
Ionisation
What is the ionisation energy equal to?
Energy needed to completely remove an electron from the ground state
How do fluorescent tubes produce light?
By using excited electrons
State the process for fluorescent tubes to produce light
- Contain mercury vapour, which accelerate free electrons under a high PD which ionise mercury atoms, producing more electrons
- When free electrons collide with mercury atoms, electrons are excited to higher energy levels
- When excited electrons return to ground states, they emit photons in UV range
- Phosphourus coating inside the tube absorbs these photons, exciting its electrons to much higher levels
- These electrons cascade down energy levels, emitting many lower energy photons in the form of visible light
What type of spectrum do fluorescent tubes produce?
Line emission spectra
What spectrum does shining white light through a cool gas give?
Absorption spectrum
What type of wavelengths do continuous spectra contain?
All types of wavelength
What spectra is white light?
Continuous
How can you test if the spectra emitted is continuous?
Split the light emitted up via a prism, and the colours should merge without any gaps in the spectrum
Why does a continuous spectrum contain all types of wavelength?
Electrons are not confined to energy levels in the object. They electrons are also not bound and are free
What do cool gases do to a continuous spectrum?
Remove certain wavelengths
In a gas at low temperatures, what state will most electrons be in an atom?
Ground state
State how a cool gas removes certain wavelengths from the continuous spectrum
- As most electrons will be at ground states, photons of corresponding wavelengths can be absorbed by electrons to excite them to higher energy levels
- These wavelengths are missing from the continuous spectrum when it comes out of the other side of the gas
What do black lines in a spectrum show?
The absorbed wavelengths
What is the similarity between an emission and absorption spectra of a particular gas
The black lines in the absorption spectrum match up to the bright lines in the emission spectrum
How can light be shown to be a wave?
By interference and diffraction
How can light be shown to be a particle?
Photoelectric effect
What does electron diffraction show?
Wave nature of electrons
How can electrons show diffraction patterns?
When accelerated in a vacuum tube and they interact with the spaces in a graphite crystal - confirms wave like properties
Do all particles show wave like properties?
No, not all the time
