Quantum Phenomena Flashcards
List three problems with classical wave theory posed by observations of photoelectricity.
I) Photoelectric emission only occurs above a certain frequency of EM radiation known as the threshold frequency.
II) The number of electrons emitted per second is proportional to the intensity of incident EM radiation only if the frequency of the EM radiation is above the threshold frequency.
III) There is no time lag between the source of radiation being turned on and electrons being emitted regardless of intensity.
What is photoelectricity?
When electrons are emitted from the surface of a metal due to incident electromagnetic radiation above a certain frequency.
How did Einstein explain the problems to classical wave theory posed by photoelectricity?
Using the idea of photons (packets of waves), Einstein said each photon can only hit one electron and so there is an instantaneous transferral of energy, explaining the lack of time lag and the problems to do with intensity.
What is the name for the amount of energy an electron needs to gain to leave the surface of a metal in photoelectric emission?
The work function Φ.
What equation concerns the threshold frequency of a metal, it’s work function and Planck’s constant?
f min = Φ/h
Describe the structure of a vacuum photocell.
A vacuum photocell is a glass tube that contains a metal plate (photocathode) and a small anode. Photons of a high enough frequency hit the photocathode and cause photo electrons to be emitted from the cathode to the anode.
Define an electron volt.
The electron volt is a unit of energy equal to the work done when an electron is moved through a pd of 1 volt.
Describe excitation by collision.
A colliding electron causes an electron of an atom to move from an inner shell to an outer shell as it now has more energy.
Describe an atom in it’s ground state and in it’s excited state.
In the ground state, an atom has has the least energy and each electron is in the lowest shell possible. In an excited state, an electron shifts to an outer shell and so the atom has more energy.
Can an atom be excited by photons?
Yes, but only if the incident photon has an energy perfectly equal to the size of a gap in energy levels of that atom.
Describe the process inside a fluorescent tube.
The fluorescent tube is a glass tube with a fluorescent coating on it’s inner surface. The tube contains mercury vapour at low pressure. When a pd is dropped across the tube the mercury atoms ionise and excite due to electrons passing through the tube. When the mercury atoms de-excite they emit photons in the UV range. This incident photons excite the atoms of the fluorescent coating, and when this coating de-excites photons in the visible spectrum are emitted.
What is a line spectrum?
A line spectrum is a spectrum of discrete lines of different colours emitted by a glowing gas or vapour.
Why do different atoms emit characteristic line spectra?
The energy levels of each type of atom are unique to that atom and so the photons emitted when the atom de-excites are unique in their combinations of wavelength and frequency to that atom.
What equation concerns the de Broglie wavelength and momentum?
De Broglie Wavelength = h/p=h/mv
