This Month in Physics History

May 1801: Thomas Young and the Nature of Light

AIP Thomas Young

The debate over whether light is a wave or a particle goes back many centuries. In the 17th century, Isaac Newton believed light was composed of a stream of corpuscles. At that time, a few scientists, most notably Dutch physicist and astronomer Christiaan Huygens, thought light was a wave vibrating in some sort of ether.

There was evidence for both pictures. For instance, sound, known then to be a wave, can travel through crooked pipes and around corners, while light cannot, and this fact was taken as evidence for the corpuscular theory of light. But phenomena such as refraction were difficult to explain with the corpuscular theory. Newton had to invoke an inexplicable force that changed the velocity of light in water. Newton was also intrigued and puzzled by colored fringes in soap films, but stuck to the corpuscular theory despite its difficulties.

Newton was so greatly revered as a scientist that it was nearly impossible for anyone to dispute his theory. In 1801 Thomas Young presented a serious challenge to Newton’s ideas on the nature of light.

Young was a true polymath, with interests ranging from physics to Egyptology. He was born in 1773 in Milverton, in southwest England, into a large Quaker family. He was a prodigy as a child, learning to read by age two, and teaching himself Latin at age six.

He began studying medicine in 1792, and was elected to the Royal Society in 1794. He was also interested in pure science. In 1801 Young was appointed to a lectureship at the recently-formed Royal Institution in London, where he gave a series of lectures on a variety of topics.

As part of his medical studies, Young had dissected an ox eye in order to figure out how the eye focuses on objects at different distances. He also proposed a theory of color vision. In addition, he was fascinated by languages, and he completed a dissertation on the human voice in which he came up with a 47 letter alphabet that covered all human sounds. His studies of the eye and ear led naturally to his interest in studying sound and light.

Young had first read Newton’s Opticks in 1790 at age 17, and had admired Newton’s work. By 1800 Young saw some problems with Newton’s corpuscular theory. For instance, he noticed that at interfaces such as that between air and water, some light is reflected and some is refracted, but the corpuscular theory can’t easily explain why that happens. The corpuscular theory also has trouble explaining why different colors of light are refracted to different degrees, Young noted.

Sound was known to be a compression wave in air; Young thought light might be similar. He noticed that when two waves of sound cross, they interfere with each other, producing beats. While he didn’t immediately look for the optical equivalent of beats, he began to realize that light might exhibit interference phenomena as well.

In May of 1801, while pondering some of Newton’s experiments, Young came up with the basic idea for the now-famous double-slit experiment to demonstrate the interference of light waves. The demonstration would provide solid evidence that light was a wave, not a particle.

In the first version of the experiment, Young actually didn’t use two slits, but rather a single thin card. He covered a window with a piece of paper with a tiny hole in it. A thin beam of light passed through the hole. He held the card in the light beam, splitting the beam in two. Light passing on one side of the card interfered with light from the other side of the card to create fringes, which Young observed on the opposite wall.  

Young also used his data to calculate the wavelengths of different colors of light, coming very close to modern values.

In November 1801 Young presented his paper, titled “On the theory of light and color” to the Royal Society. In that lecture, he described interference of light waves and the slit experiment. He also presented an analogy with sound waves and with water waves, and even developed a demonstration wave tank to show interference patterns in water.   

Despite Young’s convincing experiment, people didn’t want to believe Newton was wrong. “Much as I venerate the name of Newton, I am not therefore obliged to believe that he was infallible,” Young wrote in response to one critic. Disappointed at the response to his research on light, Young decided to focus on medicine, though he was never very successful as a physician. He did do some further work in physics, and in 1807, Young published some of his lectures, including the double- slit version of the interference experiment.

Before he died in May 1829, Young contributed to deciphering the Rosetta stone, and wrote many articles for the Encyclopedia Britannica on an incredible range of subjects, including Bridge, Carpentry, Chromatics, Egypt, Languages, Tides, and Weights and Measures.

The basic double-slit setup Young proposed has since been used not only to show that light acts like a wave, but also to demonstrate that electrons can act like waves and create interference patterns. Since the development of quantum mechanics, physicists know that light is both particle and wave, not simply one or the other.