Physics Module 4 - Electrons, Waves and Protons

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Quantum Physics

The Photon

KEY DEFINITIONS:

Quantum = a small discrete unit of energy

Photon = a quantum associated with electromagnetic radiation

A Brief History of Ideas about Light

Newton:

light is composed of a stream of tiny particles moving in a straight line from a light source

supported the theory with his own laws of motion

showed that both reflection and refraction could be explained

particles either bounce off a surface or travel more quickly as they move from a less dense to a more dense medium

Huygens:

light is a wave

wave theory could also explain reflection and refraction

light wave slows down when it passed from a less dense to a more dense medium - correct

Young:

double slit experiment showed that light behaved like a wave

passed a coherent source of light through a surface with 2 small slits, Young showed that light diffracted, overlapped amd interfered to produce bright and dark fringes of light

The Quantum Model

Planck's work on the electromagnetic radiation emitted by hot objects made him realise that the radiated energy could not have continuous values

Planck hypothesised that the total radiated energy had to be emitted in packets, with each packet, or quantum, having a definite, fixed amount of energy

The Photon Model

Broglie showed that light could behave as both a particle and a wave

Quanta of light or other electromagnetic energy are known as photons

The energy of a single photon is given by the equation E = hf

E is the photon energy in joules

h is the Planck constant, 6.626 x 10^-34 Js

f is the frequency of the radiation in Hz

The energy of a photon is directly proportional to its frequency

Photon energies are always emitted in whole number multiples

e.g. E = nhf (n = 0, 1, 2, 3...)

The Photon Model (cont.)

The equation relating the speed of light, c, the frequency of a wave, f, and its wavelength is

wave speed = frequency x wavelength

To calculate the photon energy, we use

E = hf if we know the frequency of the radiation

E = hc/wavelength if we know the wavelength of the radiation

The total energy transferred by an electromagnetic wave is equal to the sum of the energies of the photons emitted by the light source

The Electronvolt

KEY DEFINITIONS:

Electronvolt: the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt