Nature and Propagation of Light

1. Introduction

Light is a form of energy that produces the sensation of vision. The question of what light is has two historically competing answers: the corpuscular (particle) theory and the wave theory. Modern physics accepts that light has a dual nature, behaving as a wave in some experiments (interference, diffraction, polarization) and as a stream of particles (photons) in others (photoelectric effect). This chapter develops the wave picture of light and shows how Huygens' principle explains the everyday laws of reflection and refraction.

Brief historical background:

• Newton (1670s): corpuscular theory, light as tiny particles.
• Huygens (1678): wave theory, light as waves travelling through a medium ("ether").
• Young (1801): double-slit experiment demonstrated interference, strong evidence for the wave nature.
• Maxwell (1865): established that light is an electromagnetic wave.
• Einstein (1905): the photoelectric effect confirmed light also has a particle (photon) nature.

For wave optics (reflection, refraction, interference, diffraction, polarization) the wave theory of Huygens is the working model.

2. Wavefronts

A wavefront is the locus of all points of a wave that are vibrating in the same phase at a given instant. The direction of propagation of the wave (the ray) is always perpendicular to the wavefront.

The speed of light in a medium of refractive index $n$ is:

$v = \frac{c}{n}$

where $c = 3\times 10^{8}\ \text{m s}^{-1}$ is the speed of light in vacuum.

Types of wavefront:

1. Spherical wavefront: produced by a point source. The wavefronts are concentric spheres centred on the source, and the rays travel radially outward.
2. Cylindrical wavefront: produced by a line source (a long narrow slit). The wavefronts are coaxial cylinders.
3. Plane wavefront: at a very large distance from the source, a small portion of a spherical wavefront is effectively flat. All the rays are then parallel. This is the wavefront assumed in most optics problems, and a laser produces a nearly perfect plane wavefront.