Quantum Phenomena Flashcards
Describe the photoelectric effect
When electromagnetic radiation is directed at a metal, electrons can be emitted from its surface.
Rules for the photoelectric effect
Emission of electrons does not take place if the frequency of the incident EM radiation is below the threshold frequency (which depends on the metal). The number of electrons emitted per second is proportional to the intensity of the incident radiation. Photoelectric emission occurs as soon as the incident radiation is directed at the surface. Increase the frequency of the incident radiation increases the photoelectrons’ energy.
Explain the photoelectric effect
When the EM radiation is incident on the metal surface, an electron at the surface absorbs a single photon from it and therefore gains energy equal to hf. The electron can leave the surface if this energy gained exceeds the work function of the metal. Any excess energy gained by the electron is its kinetic energy.
Define stopping potential (Vs)
The minimum potential needed to stop photoelectric emission. It works by applying a positive charge to the metal plate to attract electrons back to it.
Describe the graph of kinetic energy against frequency for photoelectrons
KE directly proportional to frequency. Line crosses y axis at -work function. Crosses x axis at the threshold frequency. The gradient is h.
Describe the graph of stopping potential against frequency for photoelectrons.
Vs directly proportional to frequency. Line crosses y axis at -work function/charge of an electron. Crosses x axis at the threshold frequency. The gradient is h/e.
Define excitation
A process where an electron in an atom absorbs energy and moves to a higher energy level and the atom is not ionised.
Define de-excitation
Where an exited electron relaxes to a lower energy level. This electron emits a photon with energy equal to the energy difference between the energy levels.
When can a photon cause excitation?
When it’s energy is exactly the difference between the energy levels the electron moves from and to. Otherwise the photon will not be absorbed by the electron.
How does a fluorescent tube work?
It contains mercury vapour at low pressure. Electrons passing through the tube collide with the electrons in the mercury atoms transferring energy causing the orbital electrons to move to a higher energy level. Each excited electron relaxes to a lower energy level emitting UV and visible photons depending on the difference between the energy levels. UV photons are absorbed by the atoms in the fluorescent coating causing excitation. The atomic electrons de-excite indirectly to the previous lower level emitting visible photons.
How can we use spectra to determine elements?
Use a tube of glowing gas as the light source through a prism and a line spectrum is formed. By measuring the wavelength of each line you can identify the element which produced the light because no other element produces the same pattern of wavelengths.
Name some phenomena that suggest light is a wave
Diffraction, interference, superposition.
What is the phenomenon that suggests light is a particle?
The photoelectric effect because only particles can transfer all of their energy at once.
De Broglie’s hypothesis
Matter particles have a dual wave-particle nature. This behaviour is characterised by a wavelength (its de Broglie wavelength) which is related to momentum.
How was it demonstrated that electrons have wave properties?
A narrow beam of electrons was directed at a thin metal foil. It’s rows of atoms cause the electrons to be diffracted, but only in certain directions. A pattern of rings forms on a fluorescent screen. The bright rings are where the electrons constructively interfere like waves do.
