# aqa physics particles and photoelectric effect

• Created by: Amelia
• Created on: 28-04-14 17:47

Particle Physics Summary (chapters 1 to 3)

Light waves (pages 8 and 41)

·         Light are electromagnetic waves and they make up a section of the electromagnetic spectrum.

·         Electromagnetic waves consist of electric and magnetic waves vibrating in phase, at right angles.

·         The speed of light is described by the equation, c = f x λ

o   c = speed of light (3 x 108 ms-1), f = frequency (Hz) and λ = wavelength (m)

·         The interference pattern seen when light is passed through two slits gives evidence for the wave nature of light.

Light photons (pages 8 and 9)

·         A ‘packet of electromagnetic waves’ is called a photon and is considered to be a particle.

·         The energy of a photon is directly proportional to its frequency by E = h x f.

o   h = Planck constant (6.63 x 10-34 Js).

·         Recalling that f = c / λ , we also have an equation in terms of wavelength of E = hc / λ.

·         Evidence for light being made up of photons comes from the photoelectric effect described below.

Photoelectric effect (pages 30 and 31)

·         When light is shone on the surface of a metal, the energy can liberate electrons which are emitted from the surface of the metal.

·         Certain observations made when increasing the frequency (therefore energy) of the light cannot be explained by light being a wave.

Observation

Wave theory

Particle theory (photons)

There is a threshold frequency, below which no electron emissions are seen, regardless of the intensity of the light.

The cumulative effect of increasing the intensity should see eventually see photoelectric emissions at any frequency.

Since individual photons interact with individual electrons. If the photons do not have sufficient energy, an electron cannot be emitted.

Regardless of intensity, emissions are seen immediately when above the threshold frequency.

The cumulative effect of increasing the intensity should mean low intensity light takes longer for electron emissions to be seen than high intensity.

Since individual photons interact with individual electrons. As such a single photon with sufficient energy will immediately emit an electron.

The intensity of the light does not effect on the maximum kinetic energy of emitted electrons.

The cumulative effect of increasing the intensity should mean a higher intensity leads to electrons being emitted with a higher kinetic energy.

Since individual photons interact with individual electrons. Increasing intensity simply means more photons of the same energy, leaving the kinetic energy of emitted electrons unchanged.

·         The photoelectric effect can be described by the equation

o   hf = the energy of a photon.

o    = the work function of the metal (which is the minimum energy required to emit an electron).

o    = the maximum kinetic energy of (explaining where any excess photon energy goes).

o   When the frequency