ABSTRACT

Our understanding of the nature of light has changed back and forth over the past few centuries between two apparently conflicting viewpoints. A highly readable account of the evolution of quantum theory is given in Gribben (1984). In the late 1600s, Newton's mechanistic view of light as being made up of small particles prevailed. By the early 1800s, experiments by both Young and Fresnel had shown interference effects in light beams, indicating that light was made up of waves. By the 1860s, Maxwell's theories of electromagnetic radiation were accepted, and light was understood to be part of a wide spectrum of electromagnetic waves with different wavelengths. In 1905, Einstein explained the photoelectric effect by proposing that light is made up of discrete particles or quanta of energy. This complementary nature of light is now well accepted. It is referred to as the particle-wave duality, and is summarised by the equation https://www.w3.org/1998/Math/MathML"> E = h f = h c / λ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781849770491/73e68402-560c-4e2d-80e3-91abe8fec22d/content/math_1_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> where light, of frequency f or wavelength λ comes in ‘packets’ or photons, of energy E, h is Planck's constant (6.626 × 10−34 Js) and c is the velocity of light (3.00 × 108 m/s) (NIST, 2002).