Transmission and scattering of light

Introduction

For centuries physicists argued about whether light was a particle or a wave. It was assumed that light could only be one or the other, but not both.

In earlier chapters on waves and optics, you studied how light or other electromagnetic radiation propagates like a wave. The wave nature of light was demonstrated by the propagation of light in examples such as diffraction, interference, and polarisation of light.

You also saw in Electromagnetic Radiation how light sometimes behaves as a particle. This chapter looks at evidence supporting the particle model of light . The idea that light can have both wave and particle properties was one of the most important discoveries of the twentieth century.

The transmission and scattering of light

Energy levels of an electron

We have seen that the electrons in an atom have different energy levels. When the electron receives enough energy, it can jump up to a higher energy level. This is called 'exciting' the electron. When the electron in a high energy level sheds some energy, it drops to a lower energy level.

We have also seen that the energy associated with light at a specific wavelength is given by:

In the particle model of light, this means that each packet of light (photon) at a wavelength $\lambda$ has energy:

For the electron to receive energy, it absorbs a photon and gets its energy. When an electron loses energy to drop to a lower level, it emits (gives off) a photon with that energy.

Interaction of light with metals

When light encounters or passes through a material, the photons of the light interact with the atoms or molecules of the material. Depending on the strength of the interactions and how often they happen, the light will pass through the material or be scattered in some other direction.

Each wavelength of light relates to a particular energy, and the closer that energy is to the energy difference between two of the levels of the atom, the likelier the photon is to interact with the atom.

When visible or ultraviolet (UV) radiation shines on a metal, the photons are absorbed by the electrons in the metal. The electrons are then excited up to a higher energy level. When an electron returns to a lower energy level, another photon is emitted. This is how light appears to be reflected off a metal surface.

In previous chapters, you have studied geometrical optics, which tells us what happens to rays of light when they are reflected off a surface or refracted through a lens. That tells us what happens to light rays, made up of many photons, on a large scale. If you look at a smaller level, i.e. on a microscopic scale, then reflection and refraction happen by all the photons interacting with the atoms of the lens or mirror. The photons get absorbed and re-emitted many times before emerging as the finals rays of light that we see.

Scattering of light is responsible for many effects in everyday life. We see that certain materials are red or blue, for example, since they contain materials that have energy level differences that correspond to the energies of the photons that make up red or blue light. These materials then reflect the red or blue light and absorb the other wavelengths in the visible spectrum. White objects reflect photons of all wavelengths in the visual spectrum, while black objects absorb these photons.