The exact nature of visible light is a mystery that has puzzled man for centuries. Greek scientists from the ancient Pythagorean discipline postulated that every visible object emits a steady stream of particles, while Aristotle concluded that light travels in a manner similar to waves in the ocean. Even though these ideas have undergone numerous modifications and a significant degree of evolution over the past 20 centuries, the essence of the dispute established by the Greek philosophers remains to this day.

One point of view envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock. The opposing view holds that light is composed of a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle. During the past few centuries, the consensus of opinion has wavered with one view prevailing for a period of time, only to be overturned by evidence for the other. Only during the first decades of the twentieth century was enough compelling evidence collected to provide a comprehensive answer, and to everyone's surprise, both theories turned out to be correct, at least in part.

The Duality of Light - In the early eighteenth century, the argument about the nature of light had turned the scientific community into divided camps that fought vigorously over the validity of their favorite theories. One group of scientists, who subscribed to the wave theory, centered their arguments on the discoveries of Dutchman Christiaan Huygens. The opposing camp cited Sir Isaac Newton's prism experiments as proof that light traveled as a shower of particles, each proceeding in a straight line until it was refracted, absorbed, reflected, diffracted or disturbed in some other manner. About 200 years later, quantum mechanics was born from the research of Einstein, Planck, de Broglie, Neils Bohr, Erwin Schrödinger, and others who attempted to explain how electromagnetic radiation can display what has now been termed duality, or both particle-like and wave-like behavior. At times light behaves as a particle, and at other times as a wave. This complementary, or dual, role for the behavior of light can be employed to describe all of the known characteristics that have been observed experimentally, ranging from refraction, reflection, interference, and diffraction, to the results with polarized light and the photoelectric effect. Combined, the properties of light work together and allow us to observe the beauty of the universe.

Neils Bohr (1885-1962) - Building on Ernest Rutherford's work on the nucleus, Bohr developed a new theory of the atom, which he completed in 1913. The work proposed that electrons travel only in certain orbits and that any atom could exist only in a discrete set of stable states. Bohr further held that the outer orbits, which could hold more electrons than the inner ones, determine the atom's chemical properties and conjectured that atoms emit light radiation when an electron jumps from an outer orbit to an inner one. Although Bohr's theory was initially viewed with skepticism, it earned him the Nobel Prize in physics in 1922 and was eventually expanded by other physicists into quantum mechanics.

Albert Einstein (1879-1955) - Albert Einstein was one of the greatest and most famous scientific minds of the 20th century. The eminent physicist is best remembered for his theories of relativity, as well as his revolutionary notion concerning the nature of light. However, his innovative ideas were often misunderstood and he was frequently ridiculed for his vocal involvement in politics and social issues. The birth of the Manhattan Project yielded an inexorable connection between Einstein's name and the atomic age. However, Einstein did not take part in any of the atomic research, instead preferring to concentrate on ways that the use of bombs might be avoided in the future, such as the formation of a world government.

Christiaan Huygens (1629-1695) - Christiaan Huygens was a brilliant Dutch mathematician, physicist, and astronomer who lived during the seventeenth century, a period sometimes referred to as the Scientific Revolution. Huygens, a particularly gifted scientist, is best known for his work on the theories of centrifugal force, the wave theory of light, and the pendulum clock. His theories neatly explained the laws of refraction, diffraction, interference, and reflection, and Huygens went on to make major advances in the theories concerning the phenomena of double refraction (birefringence) and polarization of light.

Sir Isaac Newton (1642-1727) - Sir Isaac Newton, who was ironically born the same year that Galileo died, is popularly known as one of history's greatest scientists. Many of his discoveries and theories in the areas of light, color, and optics form the basis for current scientific thought in these disciplines. In addition to his extensive work in optics, Newton is perhaps best known for his theory of universal gravitation. He also is considered one of the inventors of calculus along with German mathematician Gottfried Leibniz. Newton's three laws of motion are considered basic to any physics student's education.

Max Planck (1858-1947) - Max Planck, a German physicist, is best known as the originator of the quantum theory of energy for which he was awarded the Nobel Prize in 1918. His work contributed significantly to the understanding of atomic and subatomic processes. Planck made significant contributions to science throughout his life. He is recognized for his successful work in a variety of fields including, thermodynamics, optics, statistical mechanics, and physical chemistry.

Thomas Young (1773-1829) - Thomas Young was an English physician and a physicist who was responsible for many important theories and discoveries in optics and in human anatomy. His best known work is the wave theory of interference. Young was also responsible for postulating how the receptors in the eye perceive colors. He is credited, along with Hermann Ludwig Ferdinand von Helmholtz, for developing the Young-Helmholtz trichromatic theory.

Particle and Wave Reflection - One point of view envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock. The opposing view holds that light is composed of a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle. This interactive tutorial explores how particles and waves behave when reflected from a smooth surface.

Particle and Wave Refraction - When a beam of light travels between two media having different refractive indices, the beam undergoes refraction, and changes direction when it passes from the first medium into the second. According to the wave theory, a small portion of each angled wavefront should impact the second medium before the rest of the front reaches the interface. This portion will start to move through the second medium while the rest of the wave is still traveling in the first medium, but will move more slowly due to the higher refractive index of the second medium. Because the wavefront is now traveling at two different speeds, it will bend into the second medium, thus changing the angle of propagation. In contrast, particle theory has a rather difficult time explaining why particles of light should change direction when they pass from one medium into another.

Particle and Wave Diffraction - Particles and waves should behave differently when they encounter the edge of an object and form a shadow. Newton was quick to point out in his 1704 book Opticks, that "Light is never known to follow crooked passages nor to bend into the shadow". This concept is consistent with the particle theory, which proposes that light particles must always travel in straight lines. If the particles encounter the edge of a barrier, then they will cast a shadow because the particles not blocked by the barrier continue on in a straight line and cannot spread out behind the edge. On a macroscopic scale, this observation is almost correct, but it does not agree with the results obtained from light diffraction experiments on a much smaller scale.

Thomas Young's Double Slit Experiment - In 1801, an English physicist named Thomas Young performed an experiment that strongly inferred the wave-like nature of light. Because he believed that light was composed of waves, Young reasoned that some type of interaction would occur when two light waves met. This interactive tutorial explores how coherent light waves interact when passed through two closely spaced slits.

Reference Listing - Whether light can be better described as a continuous wave or a collection of particles has challenged many great physicists over the ages, resulting in a variety of theories to support both models. The dual nature of light, which is manifested in properties that are at times consistent with either or both corpuscular (particle) and wave theories, is discussed in the review articles and books listed in this literature reference section.

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