3.6 Wave-particle duality Flashcards
Light is a part of the…
…electromagnetic spectrum of waves
What did the theory of electromagnetic waves predict
The existence of electromagnetic waves beyond the visible spectrum
The discovery of x-rays and radio waves confirmed these predictions
When is the wave-like nature of light observed
When diffraction takes place
Eg. when light passes through a narrow slit
Light emerging from the slit spreads out in the same way as water waves spread out after passing through a gap
The narrower the gap, the ______ the amount of diffraction
Greater
The longer the wavelength, the ______ the amount of diffraction
Greater
When is the particle-like nature of light observed
For example in the photoelectric effect.
When light is directed at a metal surface and an electron at the surface absorbs a photon of frequency f, the kinetic energy of the electron is increas4ed from a negligible value by hf. The electron can escape if the energy it gains from a photon exceeds the work function of the metal
What can electrons in a beam be deflected by
A magnetic field
Evidence that electrons have a particle-like nature
The fact that electrons in a beam can be deflected by a magnetic field
Who first considered the idea that matter particles also have a wave-like nature
Louis de Broglie in 1923
The de Broglie hypothesis
What hypothesis did de Broglie put out
That matter particles have a dual wave-particle nature
The wave-like behaviour of a matter particle is characterised by a wavelength, its de Broglie wavelength which is related to the momentum p of the particle
Equation of the Broglie wavelength related to the momentum p of the particle
λ = h/p
In which:
λ = wavelength
h = Planck’s constant
p = momentum of the particle
A particle of mass m moving at velocity v has a Broglie wavelength given by:
λ = h / mv
In which: λ = wavelength h = Planck's constant m = mass v = velocity
Momentum of a particle is defined as its…
…mass * velocity
When was the wave-like nature of electrons discovered
Three years after de Broglie put forward his hypothesis
It was demonstrated that a beam of electrons can be diffracted
How was evidence for de Broglie’s hypothesis obtained
Narrow beam of electrons in vacuum tube directed at thin metal foil, metal composed of many tiny crystalline regions which consist of + ions in fixed positions in rows in a regular pattern. Rows of atoms cause electrons in beam to be diffracted
Electrons in beam pass through foil and are diffracted in certain directions, form a pattern of rings on fluorescent screen at end of tube. Each ring due to electrons diffracted by same amount from grains of different orientations at same angle to incident beam
Beam of electrons produced by attracting electrons from heated filament wire to + charged metal plate which has small hole at centre. Electrons that pass through hole form beam, speed of these can be inc. by inc. p.d. between filament and metal plate. Diffraction rings smaller as inc. speed makes de Broglie λ smaller so less diffraction
In the diffraction of electrons, what happens if the potential difference between the filament and the metal plate is increased
The diffraction rings are smaller as the increase of speed makes the de Broglie wavelength smaller so less diffraction occurs and the rings become smaller
An electron in an atom has a ______ amount of energy that depends on the shell it occupies. Its __ ______wavelength has to fit the ______ and ______ of the shell. This is why its energy depends on the shell it occupies.
Fixed
de Broglie
Shape
Size
An electron in a spherical shell moves round the nucleus in a ______ orbit. The circumference of its orbit must be equal to a ______ ______ of de Broglie wavelengths (_= __ where n=1 or 2 or 3 etc,) This condition can be used to derive the energy level formula for the hydrogen atom.
Circular
Whole number
c = nλ
What is the STM (scanning tunnelling microscope) used for
To map atoms on solid surfaces.
How do STMs (scanning tunnelling microscopes) work
The wave nature of electrons allows them to tunnel between the surface and a metal tip a few nanometres above the surface as the tip scans across the surface
What is the TEM (transmission electron microscope)
Used to obtain very detailed images of objects and surface features too small to see with optical microscopes
How do TEMs (transmission electron microscopes work
Electrons are accelerated in a TEM to high speeds so their de Broglie wavelength is so small that they can give very detailed images
What is the MR (Magnetic resonance) used for
Used in hospitals, detects radio waves emitted when hydrogen atoms in a patient in a strong magnetic field flip between energy levels
What are SQUIDs (Superconducting quantum interference devices) used to detect
Very very weak magnetic fields
Eg. magnetic fields produced by electrical activity in the brain
Does the de Broglie wavelength of an electron increase or decrease when it moves to an orbit where it travels faster
The wavelength becomes smaller because its momentum becomes greater.
