The Gist of Science Reporter: August 2015

The Gist of Science Reporter: August 2015

Decoding the Engima Called Light

We see light and we see through light every day, everywhere. Over the years, scientists researching in optics - the science of light - have amassed a repository of knowledge, thanks to their myriad discoveries, which have provided us an insight into the enigma that is light. This quest to unravel the enigma can be depicted in the form of an ‘optics knowledge spiral’ shown on the next page.

The journey of light dates back to antiquity. Our journey of the optics knowledge spiral may begin through simple, light hearted queries like what is light and wondering how various phenomena in nature involving light occur such as sunrise and sunset, blues of sky and sea, rainbow, and the remarkable range of colors of plants and animals. It is estimated that since about 4.5 billion years the sun has been constantly lighting our earth by converting mass into energy. Life is believed to have originated by cyanobacteria in ocean because of light by the process of photosynthesis about 2 billion years ago. Pythagoras (582-500 BC) thought a light as particles that produce the sensation of vision upon entering the eye. Plato (427-347 BC), on the other hand, supposed that vision was produced by rays of light that originate in the eye and then strike the object being viewed. Aristotle (384-322 BC) considered light in the form of waves and Euclid (320-275 BC) postulated that light rays travel in a straight line and applied the knowledge of geometry to study paths of light. Hero of Alexandria (150 BC) is often credited with discovering the properties of reflection of light whereas Claudius Ptolemy (100-170 AD) performed elaborate experiments to measure the bending of a light beam as it passed from air into water or glass.

This early understanding of light had corresponding applications in different times. The simplest application of light is lighting. Fire constituted early man’s first use of artificial lighting in the form of flaming torch and campfire. It was followed by the use of primitive lamps made from natural sources like rocks and shells by prehistoric humans. Fireflies have also been used for illumination in the West Indian Islands and Japan. The invention of the candle dates back to about 400 A.D.

Optics since the time of the Greeks up to the end of the 18th century was based mainly on the assumption that light consists of rays that obey the geometrical laws of reflection and refraction and thus came to be known as ray optics or geometrical optics. It dealt mainly with the effect of instruments such as prisms, lenses and mirrors on the paths traced by light rays. Theory and applications of geometrical optics thus constituted the depth and breadth of the optics know ledge spiral till this time. The breadth kept expanding with aims of improving the fidelity of images produced by the optical instruments.

The first microscope was developed around 1595 by the Dutch eyeglass makers, Hans Lippershey, Hans Janssen and his son, Zacharias. The earliest known telescopes were developed in 1608 in Netherlands by Hans Lippershey, Zacharias Janssen and Jacob Metius. Galileo (beginning of 17th century) and Kepler (1610) made pioneering contributions to the refinement of a telescope. The bifocals were developed by Benjamin Franklin, an American scientist, in 1784. The pioneering research of Leeuwenhoek, a Dutch microscopist who developed the theory of aberration-free lenses by about 1830, led to improvement in magnification of lenses. By the end of the 19th century microscopes with large magnifying power were available.

Camera was improved through the pioneering work of Joseph-Nicephore Niepce and Louis Jacques Mande Daguerre of France (1820-30). The word ‘photography’ was coined by Sir John Herschel in 1839 and in 1847 Claude Niepce invented the photographic glass plate. In 1888, Eastman coined the word ‘Kodak’ and started marketing a hand-held camera, using his new film. In 1861, the Scottish physicist, James Maxwell demonstrated the first color photographs. The development of zoom lens and telephoto lens helped to refine the design and performance of the camera.

By the 20th century a variety of cameras for specific purposes such as aerial photography, document copying, movie, TV and video were developed. In addition to these many other optical instruments such as tracking instruments, gunsights, periscopes and rangefinders have been developed for different uses. Today geometrical optics has been relegated to background due to further developments in our understanding of light. However, its importance as a tool in solving practical problems is still unabated.

Light as Waves

In 1678, a Dutch physicist, Christian Huygens, suggested the wave theory of light, which assumes light to be a wave, similar to a water wave or a sound wave. This theory was highly successful due to its close agreement with experiments. The Wave Theory was substantiated in 1873 by James Maxwell who propounded that light is not only a wave but that it is a transverse, electromagnetic wave (wave of oscillating electric and magnetic fields) which can travel through vacuum.

Optics based on the assumption of light as waves came to be known as wave optics. It deals with phenomena involving light interacting with dimensions of the order of its wavelength. Theory and applications of wave optics mainly constituted the depth and breadth of the optics knowledge spiral in the 19th century. Theory concerned with the study of the patterns formed due to interference, diffraction and polarisation and applications were based on the use of this knowledge. Thus the sciences of interferometry, diffractometry and polarimetry enriched the depth, and applications of interferometric, diffractometric and polarimetric techniques (using instruments such as interferometers, diffractometers and polarimeters) augmented the breadth of the spiral.

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