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The Nature of Light

An article by Dr. David Lewis Anderson

What is Light?

Light, form of energy visible to the human eye that is radiated by moving charged particles. Light from the sun provides the energy needed for plant growth and plants convert the energy in sunlight into storable chemical form through a process called photosynthesis.

Petroleum, coal, and natural gas are the remains of plants that lived millions of years ago, and the energy these fuels release when they burn is the chemical energy converted from sunlight. When animals digest the plants and animals they eat, they also release energy stored by photosynthesis.

Scientists have learned through experimentation that light behaves like a particle at times, and like a wave at other times. The particle-like features are called photons. Photons are different from particles of matter in that they have no mass and always move at the constant speed of 300,000 km/sec (186,000 mi/sec). When light diffracts, or bends slightly as it passes around a corner, it shows wavelike behavior.

The waves associated with light are called electromagnetic waves because they consist of changing electric and magnetic fields.

Nature of Light

To understand the nature of light and how it is normally created, it is necessary to study matter at its atomic level. Atoms are the building blocks of matter, and the motion of one of their constituents, the electron, leads to the emission of light in most sources.

German-born American physicist Albert Einstein’s elegant equation E=mc2 predicted that energy could be converted to matter. Using a linear accelerator and high-energy laser light, physicists have done just that.

The following article describes their success.

Scientists Create Matter Out of Light

Creating Matter Out of Light
Physicists at the Stanford Linear Accelerator Center (SLAC) in California have succeeded in producing particles of matter from very energetic collisions of light. The team, which included researchers from Stanford University, the University of Rochester in New York, the University of Tennessee in Knoxville, and Princeton University in New Jersey, published an account of their work in the September 1, 1997, issue of the journal Physical Review Letters.

Scientists have long known that matter can be converted to energy and, conversely, energy can be converted to matter. In 1905 physicist Albert Einstein quantified the relationship between matter and energy in his famous equation E=mc2, in which E is energy, m is mass, and c is the speed of light (300,000 km/sec [186,000 mi/sec]). In an atomic bomb blast, a very small amount of matter is converted to its equivalent in energy, creating an immense explosion.

Scientists have also created matter from energy by bombarding heavy atoms (atoms made up of many protons and neutrons) with high-energy radiation in the form of X rays. Collisions between the X-ray beam and the atoms created matter in the form of sets of electron and positron particles, a phenomenon known as pair production. Positrons are particles that have the same weight and amount of charge as electrons, but positrons are positively charged, while electrons are negatively charged.

In the recent experiments at SLAC, physicists accelerated a beam of electrons to nearly the speed of light. They then aimed a split-second pulse of high-energy laser light directly at the electron beam. Occasionally a photon (a tiny, discrete unit of light energy) collided with an electron. The photon then recoiled from the collision and rebounded into oncoming photons from the laser beam with such violence that the resulting energy was converted into an electron-positron pair.

Over several months of such experiments, the physicists were able to produce more than 100 electron-positron pairs.

Light Theories

Light Theory
The earliest speculations about light were hindered by the lack of knowledge about how the eye works. The Greek philosophers from as early as Pythagoras, who lived during the 5th century BC, believed light issued forth from visible things, but most also thought vision, as distinct from light, proceeded outward from the eye.

Plato gave a version of this theory in his dialogue Timaeus, written in the 3rd century BC, which greatly influenced later thought.

Some early ideas of the Greeks, however, were correct. The philosopher and statesman Empedocles believed that light travels with finite speed, and the philosopher and scientist Aristotle accurately explained the rainbow as a kind of reflection from raindrops. The Greek mathematician Euclid understood the law of reflection and the properties of mirrors.

Early thinkers also observed and recorded the phenomenon of refraction, but they did not know its mathematical law. The mathematician and astronomer Ptolemy was the first person on record to collect experimental data on optics, but he too believed vision issued from the eye. His work was further developed by the Egyptian scientist Ibn al Haythen, who worked in Iraq and Egypt and was known to Europeans as Alhazen. Through logic and experimentation, Alhazen finally discounted Plato’s theory that vision issued forth from the eye.

In Europe, Alhazen was the most well known among a group of Islamic scholars who preserved and built upon the classical Greek tradition. His work influenced all later investigations on light.

Modern Theories

Planck’s theory remained mystifying until Einstein showed how it could be used to explain the photoelectric effect, in which the speed of ejected electrons was related not to the intensity of light, but to its frequency. This was consistent with Planck’s theory, which suggested that a photon’s energy was related to its frequency. During the next two decades scientists recast all of physics to be consistent with Planck’s theory.

The result was a picture of the physical world that was different from anything ever before imagined. Its essential feature is that all matter appears in physical measurements to be made of quantum bits, which are something like particles. Unlike the particles of Newtonian physics, however, a quantum particle cannot be viewed as having a definite path of movement that can be predicted through laws of motion.

Quantum physics only permits the prediction of the probability of where particles may be found.

The probability is the squared amplitude of a wave field, sometimes called the wave function associated with the particle. For photons the underlying probability field is what we know as the electromagnetic field. The current world view that scientists use, called the Standard Model, divides particles into two categories: fermions (building blocks of atoms, such as electrons, protons, and neutrons), which cannot exist in the same place at the same time, and bosons, such as photons, which can (see Elementary Particles).

Bosons are the quantum particles associated with the force fields that act on the fermions. Just as the electromagnetic field is a combination of electric and magnetic force fields, there is an even more general field called the electroweak field. This field combines electromagnetic forces and the weak nuclear force. The photon is one of four bosons associated with this field.

The other three bosons have large masses and decay, or break apart, quickly to lighter components outside the nucleus of the atom.