What is Light?

 

Introduction

The Merriam-Webster dictionary has 16 definitions for the word ‘light’. Which include:

 

  • Something that makes vision possible

  • A source of light, such as a celestial body

  • A spiritual illumination

 

Although all three of these areas are under consideration when looking at the use and intention of light as function and as phenomenon, this section is looking at how our understanding of light has developed over time.

 

Light is, therefore, in this context, something that makes vision possible. Light sources, both manufactured and natural, will be explored further in the ‘light as function’ and ‘light as phenomena’ sections of this blog, and spiritual illumination may briefly be touched upon.

 

The history of human understanding of light, in a scientific manner, is known as Optics, or Optical Science, and is something that has been explored since prehistoric times.

 

 

Historic Development of Optical Science

The development of Optical Science and humanity’s interest in understanding light began with the development of transmissive optical devices, known as lenses, in the Ancient Egyptian and Mesopotamian periods.

 

What is thought to be the earliest known lens, the Assyrian Nimrud/Layard lens, created in roughly 750 BCE is perhaps the objective starting point in our understanding of light.

 

There is, however, some debate as to the legitimacy of this lens, with some critics stating that the Nimrud/Layard lens is simply a crude ornament or an unfinished attempt at creating a lens (Sines & Sakellarakis, 1987).

 

The Ancient Egyptians felt a spiritual connection to the celestial light of the sun and moon, creating temples and structures, such as the Temples of Karnak at Luxor and the temples of Rameses the Great at Abu Simbel, to bring the sun’s light inside where it could be felt as well as seen (Govan & Kim, 2013).

 

In the centuries that followed, Greek philosophers and mathematicians, such as Plato, Aristotle and Euclid, tried to understand how light was seen.

 

It was, however, the Greek philosopher Empedocles (c.490-430 BCE), who along with stating that everything was made of the four elements (fire, earth, air and water), expressed the view that it is in fact the eyes that emits rays of light. This view, now termed the ‘extromissive theory’ was held until approximately the Twelfth century (Zajonc, 2013).

 

One of the other earliest known writers on the subject of Optical Science, Al-Kindi (c.801-873 CE), reasoned that light is incoherent, a view in opposition to that which his predecessors held (Borys, 2004).

 

Al-Kindi's views, that it is not the eye that emits rays of light, but everything else in the world, influenced the next generation of Optical Scientists, namely; the Arabic astronomer, physicist and mathematician Ibn-al-Haytham, the former Archdeacon of Leicester Robert Grosseteste and the Thirteenth century Oxford scholar Roger Bacon (Lindberg, 1976).

Grosseteste, a teacher at the University of Oxford in the early Thirteenth century, wrote a number of scientific treatises in the 1220s and 1230s, particularly focusing on Optical Science, with works including De Luce (On the “metaphysics of light”) and De iride (On the rainbow).

 

Grosseteste’s approach, essentially Aristotelian in practice and with regard to his study of rainbows and the formation of light and colour, led him to discuss four types of light; direct, reflected, refracted and accidental (Eastwood, 1968). Grosseteste, in turn, influenced another Oxford scholar, Roger Bacon. Both Franciscans, Bacon and Grosseteste both saw the study of light as the study of God (Oliver, 2004).

 

A major milestone in the development of Optical Science came in the late Seventeenth and early Eighteenth century with the prismatic experiments of Sir Isaac Newton. Building on the work of scientists such as Johannes Kepler (1571-1630), Willebrord Snellius (1580-1626), René Descartes (1596-1650), Christiaan Huygens (1629-1695) and Robert Hooke (1635-1703), Newton published the seminary ‘Opticks’ in 1704.

 

Newton’s discoveries, by using a prism to refract light, showed that white light is comprised of all colours. He also went on to argue that light is corpuscular, a theory later countered by the English polymath Thomas Young. Young (1773-1829) showed that a wave theory, not a corpuscular one, was required to explain light.

 

The Scottish scientist, and pioneer of colour photography, James Clerk Maxwell (1831-1879) expanded on this demonstrating that the waves are ‘vibrations of electrical and magnetic fields’, or ‘electromagnetic waves’ (Lauson, Wagner & Ball, 2013).

 

At the turn of the Twentieth century, Albert Einstein (1879-1955) produced a ‘quantum’ view of light, showing that light can both be wave-like and particle-like, depending on how it is observed (Launson, Wagner & Ball, 2013).