The nature of light is a subject of no material
importance to the concerns of life or to the
practice of the arts, but it is in many other respects extremely interesting.

Thomas Young (1773-1829)

The nature of light - whether light is made of particles or is a wave - was one of the most intriguing questions of the 17th and 18th centuries, and it led to some rather bizarre consequences. The wave-particle controversy of the last few centuries has now been replaced by wave-particle duality; but this apparent combination of the two differing ideas has, in many ways, added to the complexity of the problem. While, today, we can account for many of the apparently conflicting phenomena observed, it does not mean we have established a complete theory; on the contrary, it has only served to help us understand our limitations.

During the 18th century the corpuscular or particle theory of light was favored by most scientists. Newton had expressed some difficulties with a wave theory, particularly the fact that diffraction (or 'bending') of light was not so easily observed as it was for other types of waves, for example, sound [1] and water waves. Although he did not reject completely the idea of a periodic disturbance, i.e., a wave motion, he advanced a particle theory which, because of his great authority among his contemporaries, was widely accepted. However, several of his contemporaries, notably Robert Hooke (1635-1703) and Christian Huygens (1629-1695), suggested wave theories; Hooke to account for the colors in thin films and Huygens to account for the finite velocity of light that had been determined by Olaus Roemer (1644-1710), based on the eclipses of the Jovian moons by Jupiter. Although Huygens idea of wave propagation was correct he believed incorrectly that the oscillations take place in the direction of the propagation, i.e., longitudinal waves, whereas they are, in fact, transverse waves with the oscillations perpendicular to that direction. Francesco Grimaldi (1618-1663), on the basis of some diffraction experiments using small apertures, had also suggested that light had wave-like properties. The evidence was purely qualitative and Newton's reputation was such that the corpuscular theory was not replaced by the wave hypothesis until the experiments of Thomas Young. Young's experiments were highly supportive of the wave-like nature of light and so cast serious doubts on the corpuscular theory.

Ironically, Newton had written about a curious phenomenon he noticed; of colored "rings" formed when he placed two lenses on top of each other, viz:

"I forbore to treat of these Colors, because they seemed of a more difficult Consideration, and were not necessary for establishing the Properties of Light there discoursed of."

Contrary to his assertions, Newton's rings represent one of the best proofs of the wave nature of light- as we will see - the truth of which Newton did not want to recognize!

Young was a precocious child who could read fluently at the age of two and read widely the classics. He started Latin at six, was tutored privately at first but later attended private schools. By the time he was sixteen he was proficient in Greek and Latin and was well acquainted with eight other languages, classical and modern. By the age of eighteen he was recognized as a truly accomplished scholar.

In 1792, at age nineteen, Young decided on a career in medicine. The following year he presented a paper before the Royal Society in which he attributed the accommodation of the eye to its muscular structure; he was elected one year later to membership of the Society. After completing his medical studies at Edinburgh and Göttingen, he returned to London to practice but continued his scholarly studies at Emmanuel College, Cambridge. He became financially independent on the death of an uncle and that allowed him to pursue his real interests. Some investigations on sound and light, which he carried out in 1798, likely formed the starting point for his theory of interference. In fact, his interests and contributions were so legion that he made some anonymously to avoid the charge that he was neglecting his professional duties!

In 1801 Thomas Young was appointed professor of natural philosophy at the Royal Institution, which provided him the opportunity of presenting lectures to popular audiences. Apparently, his lectures were not well suited to this kind of audience, being designed more for the specialist that the layman. He was appointed foreign secretary to the Royal Society in 1802, a post that he held to the end of his life. He resigned his professorial position at the Royal Institution, feeling that his duties were affecting his medical career. The same year he received the MB degree from Cambridge, and five years later, the degree of MD.

It was during this period that Young conducted his now-famous experimental investigations on light. In 1800 he published his Experiments on Sound and Light in the Philosophical Transactions of the Royal Society and presented a detailed account of his theory of interference in the Bakerian Lecture On the Theory of Light and Colors in 1801 [2]. In another Bakerian Lecture in 1803 entitled Experiments and Calculations Relative to Physical Optics he summarized his observations on interference and added several new phenomena. The importance of his work was not apparent to his contemporaries and his principle of interference remained more or less obscure for another fourteen years, when it was 'rediscovered' by Fresnel. Young made other significant contributions to physical optics in the areas of double refraction and dispersion.

As I mentioned previously, Young's interests were many and diverse. For example, he was the first to assign the term energyto the quantity mv² and to put work done, which he defined as (force x distance), proportional to energy. Also he introduced absolute methods for determining the elastic properties of materials - such as the Young's modulus that relates the increase in length of a wire to the force applied - and he developed the most comprehensive theory of tides then available. His contributions to archeology and philology were equally impressive as were his researches in medicine. He could make himself welcome in almost any scholarly activity and welcome the challenge it offered. He retired from active medical practice in 1814 to devote himself full-time to his scientific studies, continuing to his death in 1829. A colleague at the Royal Institution [3] said of him:

"... Had he limited himself to any one department of knowledge, he must have been the first in that department. But as a mathematician, a scholar, a hieroglyphist, he was eminent, and he knew so much that it was difficult to say what he did not know."

He described his discoveries on the interference of light in the Bakerian Lecture (November 24, 1803), in the Philosophical Transactions of the Royal Society, 94, (1804), and from lecture 39 of his Course of Lectures. In these works Young describes his experiments on diffraction, or the bending of light, and the formation of light and dark fringes from narrow slits. However, he is arguably best remembered for his 'classic' double-slit experiment that seemed to establish, without question, that light is a wave motion. He showed that light and dark fringes could be produced on a screen some distance from a pair of very closely spaced, narrow slits and were due to constructive and destructive interference of different rays. He also described a possible application; the diffraction grating that is very much in current use for selecting monochromatic, i.e., single wavelength, light, viz:

"These colors may be easily seen, in an irregular form, by looking at any metal, coarsely polished, in the sunshine; but they become more distinct and conspicuous, when a number of fine lines of equal strength are drawn parallel to each other, so as to conspire in their effects."

He explained the formation of colored bands in soap films:

"Thus, when a film of soapy water is stretched over a wine glass, and placed in a vertical position, its upper edge becomes extremely thin, and appears nearly black, while the parts below are divided by horizontal lines into a series of colored bands ... "

and Newton's rings:

"... and when two glasses, one of which is slightly convex, are pressed together with some force, the plate of air between them exhibits the appearance of colored rings, beginning from a black spot at the center, and becoming narrower and narrower, as the curved figure of the glass causes the thickness of the plate of air to increase more and more rapidly. ..."

establishing that there is a 180^o change of phase when light is reflected from the surface of a denser medium, e.g., light traveling in air reflecting from the surface of glass or metal.

It appears that Thomas Young's demonstration of the interference of light made little impression when he announced it in 1803. It took another decade of skillful studies and experiments by Augustin Fresnel (1788-1827) to convince even the staunchest of Newton's supporters that light was a wave motion.

FOOTNOTES

[1] Have you wondered how it is possible that you can hear a sound that was made round the corner of a building or behind a tree? The only explanation is that sound waves are 'bent' around objects.

[2] Young published most of his results in his Course of Lectures on Natural Philosophy and the Mechanical Arts, 1807.

[3] Sir Humphrey Davy (1778-1829).

REFERENCES

Books

M. Shamos Great Experiments in Physics (Dover Publications Inc., New York, 1987).