Rainbows and light

Rainbows result from light encountering raindrops in the presence of an observer. The phenomenon of rainbows offers many clues as to the nature of light.

• Light is a form of radiation, a type of energy that travels in the form of electromagnetic waves and can also be described as a flow of particle-like ‘wave-packets’, called photons.
• Radiation, electromagnetic waves and photons are all concepts that are interchangeable with the more general concept of light.

Theories of light

There are four principal theories that underpin our understanding of the physical properties of light as it relates to rainbows:

• Wave theory – the idea that light is transmitted from luminous bodies in an undulatory wave-like motion.
• Particle theory – the idea that the constitution and properties of light can be described in terms of the interactions of elementary particles.
• Electromagnetic theory – the classical theory of electromagnetism that describes light as coupled electric and magnetic fields, transporting energy as it propagates through space as a wave. The energy is stored in its electric and magnetic fields and can be measured in terms of its intensity.
• Quantum theory – explains the interactions of light with matter (atoms, molecules etc.) and describes light as consisting of discrete packets of energy,  photons. Quantum physics suggests that electromagnetic radiation behaves more like a classical wave at lower frequencies and more like a classical particle at higher frequencies, but never completely loses all the qualities of one or the other.

These theories tell us things about the properties of light

• Light is electromagnetic radiation, the force carrier of radiant energy.
• Whilst it carries energy and has momentum, light has no mass and so is not matter.
• Light is the result of the interaction and oscillation of electric and magnetic fields.
• Light is a microscopic phenomenon that needs macroscopic metaphors such as waves and particles to describe it.
• Once emitted at its source, light can propagate indefinitely through a vacuum in a straight line at the speed of light (299,792,458 metres a second) but can be deflected by gravity.
• In any specific instance, light can be described in terms of the inter-relationship of its wavelength, frequency and energy.
• Light slows down and is deflected as it propagates through air, water, glass and other transparent media as photons interact with matter.

Phenomena associated with light include:

• Absorption
• Diffraction
• Dispersion
• Interference
• Photoelectric effect
• Polarization
• Reflection
• Refraction
• Scattering
• Transmission

Some facts about electromagnetic waves

• An electromagnetic wave carries electromagnetic radiation.
• Electromagnetic radiation is measured in terms of the amount of electromagnetic energy carried by an electromagnetic wave.
• Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields propagating at the speed of light in a vacuum.
• The kinetic energy carried by electromagnetic waves is often simply called radiant energy or light.
• Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
• Other terms for the amplitude of light are intensity and brightness.
• Another term for the speed at which light travels is its velocity.
• We can feel electromagnetic waves release energy when sunlight warms our skin.
• The position of an electromagnetic wave within the electromagnetic spectrum can be identified by its frequency, wavelength or energy.

Some facts about photons

• Photons are the elementary building blocks and so the smallest unit used to describe light.
• Photons are the carriers of electromagnetic force and travel in harmonic waves.
• Photons are zero mass bosons.
• Photons have no electric charge.
• The amount of energy a photon carries can make it behave like a wave or a particle. This is called the “wave-particle duality” of light.