An example of the wavelike nature of light is diffraction.
- This happens when light passes through a narrow slit.
- The light emerging from the slit spreads out in the same way as water waves spread out after passing through a gap.
- The narrow the gap or the longer the wavelength, the greater the amount of diffraction.
An example of the particle-like of light is the photoelectric effect.
- When light is directed at a metal surface and an electron at the surface of absorbs a photon of frequency f,
- The kinetic energy of the electron is increased from a negligible value of hf.
- The electron can escape if the energy it gains from a photon exceeds the work function of the metal.
Electrons behave as particles when:
They are deflected in electric and magnetic fields.
De broglie wavelength eq.
λ = h/p = h/mv
Three years after de Broglie put forward his hypothesis, electron diffraction was demonstrated.
- Following on from this, it was discovered that other particles could also be diffracted.
- Knowledge and understanding of the nature of matter has changed with time.
- Changes have to be evaluated and validated by peer review within the scientific community.
- They will look for repeatable, reliable results to confirm any new hypothesis before a previous one is amended.
A narrow beam of electrons in a vacuum tube is directed at a thin metal plate foil.
- A metal is composed of many tiny crystalline regions.
- Each region, or green, consists of positive ions arranged in fixed positions in rows in a regular pattern
- These rows of atoms cause electrons in the beam to be diffracted, just as a beam of light is refracted when it passes through a slit.
The electrons in the beam passes through the metal foil:
- And are acted in certain directions only. They form a pattern of rings on a fluorescent screen at the end of the tube.
- Each ring is due to electrons diffracted by the same amount from grains of different orientations, at the same angle to the incident beam.
How is the beam of electrons made?
The beam of electrons is produced by attracting electrons from a heated filament wire to a positively charged metal plate, which has a small hole at a centre. Electrons that pass through the hole form the beam.
The speed of the electrons can be can be increased by:
Increasing the potential difference between the filiament and the metal plate. This makes the diffraction rings smaller, because the increase of speed makes the de broglie wavelength small. So less diffraction occurs and the rings become smaller.
