Electromagnetic radiation and quantum phenomena Flashcards
When will a metal emit electrons?
When you shine radiation of high enough frequency onto the metal’s surface (frequency greater than or equal to threshold =emitted)
What is the frequency that metals emit electrons?
Usually in the UV range
What happens when you shine radiation on a metal?
- Free electrons on/near the surface will absorb the radiation’s energy, making them vibrate.
- If the electrons absorb enough energy, its bonds will break and it’ll be released.
What are emitted electrons called?
Photoelectrons
What conclusions can you get from the photoelectric effect?
- No electrons emitted below the threshold frequency
- photoelectrons emitted with a variety of Kinetic Energies (0 to Max). It increases with frequency.
- Intensity is the amount of energy hitting an area per second ( Max KE is unaffected by intensity)
- Number of photoelectrons emitted per second is proportional to intensity.
Explain what wave theory says about threshold frequency. Why is this wrong?
- Wave theory says for a particular frequency of EM wave, energy carried is proportional to intensity.
- Energy is evenly distributed over the wavefront.
- This means that if shone on a metal each electron would gain a bit of energy and would eventually gain enough to be emitted (lower frequency = longer to leave)
- However, this doesn’t happen as no electrons emitted below the threshold frequency.
Explain what wave theory says about Kinetic Energy of photoelectrons. Why is this wrong?
- Higher intensity = more energy to electrons
- This means KE of electrons should increase with intensity
- However, wave theory cant explain why KE is dependent on frequency in the photoelectric effect.
What did max planck suggest about EM waves?
They are only released in discrete packets/quanta
What is the equation for energy of an EM wave?
E = hf = hc/wavelength
What did Einstein suggest about EM waves?
- EM waves (and their energy) only exist in discrete packets called photons
- And that they have a 1 on 1 interaction with the electrons on a metal’s surface
- They would transfer all their energy to that electron
- This is used to explain the photoelectric effect
Learn electroscope with metal diagram
Uv shone on metal = electrons leave gold leaf and gold leaf falls towards negative electroscope
What is the work function?
The minimum energy gain to be emitted (breaking attractions to metal)
What is a photon?
Discrete wave packet of em waves
Why are photoelectrons emitted with a range of KEs?
Because of energy losses (work done to get to surface)
What is KE affected by?
Unaffected by intensity
Increases with frequency
What is no emitted per second proportional to?
Intensity
Threshold frequency?
Work function divided by h
What happens when external pd applied?
Electrons lose there energy by doing work against pd
What is stopping potential?
Pd needed to stop electron with Ek(max)
eVs = Ek(max)
What is an eV?
The KE carried by an electron after being accelerate from rest through a pd of 1 volt (energy gain = accelerating voltage)
What happens when electron dexcited?
Emits photon equal to the difference between energy levels (vice versa with excitation)
White light through a cool gas?
Cool gas has electrons in ground state at low temps, they are excited by photons.
If put through a prism produces a continuous spectra ( absorbed wavelength are black lines)
Fluorescent tube through a prism?
Line emission spectra (bright lines = specific frequency of light)
What are line emission spectra and continuous spectra evidence for?
Discrete energy levels (only certain photons absorbed)
Fluorescent tube?
1) FT with mercury vapour, high voltage applied
2) free electrons are accelerated and they ionise some mercury atoms producing more free electrons
3) free electrons collide with mercury atoms exciting there electrons
4) when they de excite they emit high energy photos in uv range
(Range of energies and wavelengths due to different energy level transitions)
What shows that light acts as a particle and a wave?
Photoelectric effect and electron diffraction
Electron diffraction?
High velocity electrons in diffracted by spaces in graphite crystals:
1) greater spread of greater wavelength if wave theory is correct but it increases as accelerating voltage decreases
2) particles with same speed but greater mass = more tightly packed pattern
Why are electron microscopes good for looking at DNA?
Electrons waves don’t blur images as much (as they have a short wavelength and so less diffraction occurs)
