This chapter of the video:
• uses the double-slit experiment to review the behaviour of classical particles, water waves, and light.
• shows that classical particles are localized. They pass through the slits as individual particles and strike a wall in a familiar and predictable distribution.
• shows that water waves are spread out. The slits act as individual sources that produce an interference pattern as the waves overlap.
• examines the results of Young’s double-slit experiment to support the use of a wave model for light.

The world of classical physics is relatively straightforward. There is matter and energy, particles and waves. Phenonmena can be described completely as one or the other.

CLASSICAL PARTICLES
In classical physics, matter is made up of particles. The particles are localized, which means their location can be described exactly and they can only be in one place at one time. Localized particles follow trajectories that can be predicted with mathematics using variables such as velocity, acceleration, etc. When two particles are in the same place at the same time they collide and their trajectories change. Careful measurements of a particle’s location and trajectory allow us to make very precise predictions about the outcome of any event.

CLASSICAL WAVES
Energy can be transferred through a medium by the propagation of a wave. A wave is a disturbance that spreads out through the medium by making the particles of the medium move about an equilibrium position. These particles are localized and can only be in one place at one time. When two (or more) waves meet, the medium will add the amplitudes of the waves together and produce a superposition of the waves. Superposition of two (or more) waves can produce an interference pattern and this pattern can be described using geometry.

where


LIGHT AS A WAVE
In 1803, Thomas Young described the interference of light using several experiments. The experiment that has survived with his name associated with it is the double-slit experiment, in which light shines through two narrow slits. His analysis of the pattern used the geometry of wave interference, and his conclusion was that light must be some sort of wave phenomenon. Young’s double-slit experiment seemed to settle the debate about the nature of light in favour of Huygens’s wave model.