Wave-Particle Duality

The Legacy of Max Planck Before planck, light had been shown to be wave motion. Light does things that only waves do E.g. diffraction- this is where light spreads out when it passes through a single slit

Diffraction at a slit Only waves behave like this A diffraction pattern is produced when laser light passes through a fine slit

Interference Waves interfere with each other, they can reinforce each other or cancel each other out

Interference Waves interfere with each other, they can reinforce each other or cancel each other out

After Planck Planck had found that light under certain circumstances behaved as a stream of particles called photons. These particles are as real as electrons. The wave and particle nature of light are both real. We need both ideas to explain the behaviour of light. This is called wave-particle duality

De Broglie It was not long before De Broglie asked this question: Is it possible that tiny particles like electrons could have wave behaviour? Could they even have a wavelength?

Electrons in atoms Electrons which orbit nuclei are a particular problem. Theory said they should not do this. Any accelerating electron should release energy as photons and should spiral into the nucleus

The electron as a wave De Broglie suggested that the stable orbits around an atom corresponded with whole numbers of waves. The regions between the orbits could not be occupied because a whole number of waves would not fit.

The De Broglie Wavelength Theory led De Broglie to an equation for the wavelength of an electron λ is the wavelength associated with the electron h is planck’s constant mv is the mass x the velocity of the electron ( this is called the momentum of the electron) The equation applies to all particles but only very small particles like electrons have measurable wavelengths.

Electron Diffraction If electrons have a wavelength theey should behave as waves under the right circumstances. For instance they should exhibit effects like diffraction and interference.

Electron diffraction The wave nature of electrons is well established and allows us to build electron microscopes This was the first electron diffraction pattern ever photographed. The “gaps” necessary are so small that they are in fact the gaps between atoms in a crystal lattice. Which use the wave nature of the electron to produce images