CLASSICAL PHYSICS
• Classical physics is the physics of the motion, energies, and interactions of objects in the everyday world around us.
• In the double-slit experiment, tennis balls and all other classical particles move as localized particles through the slits and once they hit the screen they produce the following distribution:

• If we use water waves, sound, or any other classical waves they spread out behind the double-slit barrier and produce an interference pattern.
• Light also spreads out behind the double-slit barrier and produces an interference pattern.

QUANTUM PHYSICS
• Quantum physics is revealed in the physics of isolated processes, typically with very small subatomic objects.
• In the electron double-slit experiment, each electron hits the detection screen as a particle.
• After many electrons hit, an interference pattern forms, demonstrating wave behaviour.
• The same interference pattern forms even when we fire electrons one at a time.

• These results show that electrons exhibit both wave and particle behaviour, i.e., wave–particle duality.
• The de Broglie wavelength describes the wave behaviour of particles such as electrons. It is given by the equation

• Light also exhibits wave–particle duality. In the double-slit experiment light hits the detection screen as an individual particle, but over time it forms an interference pattern like a wave.
• A particle of light is called a photon and its energy is given by

• All quantum objects, including protons, neutrons, atoms, and molecules, exhibit wave–particle duality.
• When we look at the electron to see what it is doing while passing through the double-slit barrier, we are making a measurement which perturbs the electron and destroys the interference pattern. This demonstrates measurement disturbance.
• We can predict the overall behaviour of the electrons in the double-slit experiment, but nobody really knows what the electrons are doing between the source and the detector. To complete the picture, physicists have proposed various interpretations, including:

i) thinking of electrons as spread-out waves that collapse to point-like particles once they are measured (Collapse Interpretation),
ii) thinking of electrons as particles that are guided by an invisible wave (Pilot Wave Interpretation),
iii) thinking of parallel universes that come into being when we make measurements at the quantum level (Many Worlds Interpretation),
iv) thinking exclusively about the direct results of measurements (Copenhagen Interpretation).

• In spite of these differing views, quantum physics plays a crucial role in a number of everyday technologies including computers, remote control devices, lasers, and cell phones.