Chapter 7 Flashcards
Explain the photoelectric effect
1) Free electrons on the surface of the metal absorb energy from the light(Has to be in U.V range), making them vibrate. 2)If an electron absorbs enough energy, the bonds holding it to the metal break and the electron is released. 3) The electrons emitted are called photoelectrons.
What are the three conclusions from the photoelectric effect experiment?
Conclusion 1 - For a metal, no photoelectrons are emitted if the radiation has a frequency below a certain value- called the threshold frequency. Conclusion 2 - The value of kinetic energy the electron has when leaving the metal increases with the frequency of the radiation, and is unaffected by the intensity of the radiation. Conclusion 3 - The number of photoelectrons emitted per second is proportional to the intensity of the radiation.
What quanta?
the discrete packets of EM energy carried by EM waves.
What are photons?
wave-packets of energy from EM waves.
What is the equation for E, the energy carried by one of the wave-packets?
E=hf=hc/wavelength
h= Planck’s constants = 6.63 x 10-34 Js
f= frequency Hz
W = Wavelength m
c = speed of light in a vacuum
When a photon hits another particle how much of its energy will it transfer?
Either transfer all or none of its energy.
What is the photon model?
1) When ligh hits its suface, the metal is bombarded by photons.
2) If one of these photons collides with a free electron, the electron will gain the energy equal to hf.
What happens when you heat gas to high temperature and then stop heating it up?
If you heat gas to a high temperature, many of its electrons move to higher energy levels.
As they fall back to the ground state, these electrons emit energy as photons.
Explain how you can get a spectrum from a hot gas?
If you split the light from a hot gas with a prism or a diffraction grating you get a line spectrum. A line spectrum is seen as a series of bright lines against a black background.
1) Eath line on the spectrum corresponds to a particular wavelength of light emitted by the source. Since only certain photon energies are allowed, you only see the corresponding wavelengths.
What is a continuous spectra?
Its light that contains of all possible wavelengths.
- The spectrum of white light is continous
- If you split the light up with a prism, the colours all merge into each other - there anre’t any gaps in the spectrum.
What happens when a continuous spectrum of light (white light) passes through cool gas?
You get a line absorption spectrum.
Explain why cool gases remove certain wavelengths from the continuous spectrum.
At low temperatures, most of the electrons in the gas atoms will be in their ground state.
- Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels.
- These wavelengths are the missing from the continous spectrum when it comes out the other side of the gas.
- You see a continous spectrum with black lines in it corresponding to the absorbed wavelengths.
What was Feynmans idea of how photons get from a source to a detector?
A photon will take all of the possible paths to the detector in one go.
How can you work out the probability that a quantum will arrive at a point?
Probability ~ (Resultant phasor)2
You can find the probability that a quantum will arrive at a point from squaring the resultant phasor amplitude.
Explain how the sum over paths rule shows you the paths nearest to the quickest path were the phasors line up for a situation in which a photon is fired at a mirror so that it reflects and hits a detector.
What you find is that the paths nearest to the quickest path have phasors that almost line up, giving most of the amplitude of the resultant- and so most of the probability that the photon will reach the detector.
The final phasors for slower, longer paths near the ends of the mirror tend to “curl up” and almost cancel themselves out. They end up adding almost nothing to the resultant amplitude or probability.
