quantum theory

Explain how young’s double slit provides evidence for the wave model of light

Light passing through two narrow slits produces two coherent beams (same amplitude, phase and wavelength) that overlap on a screen to form light and dark fringes.

• bright fringes form when waves meet in phase (crest and crest or trough and trough) - constructive interference

• Dark fringes form when waves meet out of phase (crest and trough) - destructive interference

The interference pattern shows light undergoes interference and diffraction, both wave properties

Describe light as an electromagnetic wave produced by an oscillating electric charge

Light is an electromagnetic wave that is produced by an oscillating electric charge that produces mutually perpendicular, oscillating, synchronised electric and magnetic fields that propagate through empty space at the speed of light in a vacuum

Explain the concept of black-body radiation; identify that it provides evidence that electromagnetic radiation is quantised into discrete values

Black-body radiation refers to the electromagnetic radiation emitted by an object that absorbs all incident radiation & reflects none. As a black-body is heated, it emits radiation due to the vibration Of charged particles within it. The emitted spectrum depends only on temperature, hotter objects emit more energy and have shorter wave lengths, described by wien’s law.

Classical physics predicted that the intensity of radiation would increase infinitely at short wavelengths (the ultraviolet catastrophy), but experimental data showed it drop off. Plank resolved this by proposing that electromagnetic energy is quantised, emitted in discrete packets known as quanta. This proved that EM radiation is quantised.

Describe concept of a photon

A photon is a quantised packet of EM energy with energy given by E=hf

Describe the photoelectric effect in terms of the photon

The photoelectric effect occurs when photons of sufficient frequency strike a metal surface, causing the ejection of electrons. This phenomenon demonstrates that light energy is quantised into discrete packets of electromagnetic energy (photons).

Below the threshold frequency, no electrons are emitted, regardless of intensity. Above it, the effect occurs instantly as only a single photon is required to transfer all its energy to an election.

Increasing intensity increases number of photoelections ejected.

Increasing frequeny increases maximum kinetic energy of photoelectrons. Supporting particle theory of light.

Define threshold frequency (fo)