Diffraction And The Nature Of Light Flashcards
What is diffraction
Diffraction is the spreading out of a wave as it passes through a gap
What criteria must be met for a maximum de fraction to occur
The size of the gap must be of the same magnitude as the wavelength of the wave
What happens if the gap is much smaller than the wavelength of the wave
The wave will be reflected
State the diffraction grating equation
N λ=dsinø
What does electron diffraction provide evidence for
The wave nature of electrons. It suggests that particles can demonstrate wavelike properties
Describe the diffraction patter produced by electrons
Concentric circles of bright and dark fringes from a central bring point
If electrons didn’t have a wave nature, describe the pattern that would be produced when they pass through a slit
The electrons would be unaffected by the gap and pass straight through it. A single bright région would be formed
What is the name given to the wavelength of a particle
De broglie wavelength
What two factors does the de broglie wavelength depend on
1) mass
2) velocity
State the equation used to calculate the de broglie wavelength
λ=h/mv
What can ‘mv’ be replaced with in the de broglie wavelength equation
P (momentum)
What is the basic process of a pulse-echo technique?
-wave pulse emoted
-it is transmitted and reflected at a boundary
-the returning wave is detected
-the speed and time taken are used to calculate the distance
Suggest two things that may limit the amount of information that can be obtained by a pulse-echo technique
1) the wavelength of the radiation
2) the duration of the pulse
What are the two models that can be used to describe electromagnetic radiation
1) the wave model
2) the particle model
Which model does the photoelectric effect provide evidence for
The particle model
Outline the photoelectric effect
-Light is shone on a metal plate
-if the light is above a certain frequency, electrons are emitted from the surface of the metal
-if the frequency is too low, no electrons are emitted
What are the particles of light used to explain the photoelectric effect called
Photons
How do you calculate the energy of a photon
E=hf
Explain how a photon can liberate an electron
One photon interacts with one electron and transfers all its energy to it. If this energy is greater than the metals work function, the electron will have sufficient energy to be released
What is threshold frequency
A metals threshold frequency is the minimum frequency that a photon requires to liberate an electron from its surface
If the intensity of light being shone on a metal increases, how does the energy of the photoelectrons change
The energy remains unaffected. An increase in intensity means more photons per area and so more photoelectrons are emitted
Why are photoelectrons emitted with a range of kinetic energies
The electrons are at different depths in the metal so require different amount of energy to be liberated. The excess energy from a photon once an electron has been liberated, in the kinetic energy of the electron
State the equation for the maximum kinetic energy of a photoelectron
0.5mv^2=hf-ø
What is the conversion factor between eV and J
1eV=1.6x10-19J
What happens when electrons transition between energy levels
-if electrons move to a higher energy level, radiation must be absorbed
-if electrons move to a lower frequency, radiation must be emitted
Why can only certain frequencies of radiation be absorbed by an atom to cause an electron transition
The electrons can only exist in discrete energy levels. The energy of the photon absorbed must be the exact amount of energy required to cover the difference between two discrete energy levels
