## Nature of Light

What is light, that wonderful swift carrier of information without which we cannot investigate the depths of the universe nor the secrets of the microworld? For Newton, light was made of particles while Huygens regarded light as waves in a hypothetical medium, the ether. Thomas Young solved the question once and for all, or at least this is how it appeared.

Young started his career in medicine which he studied in London, Edinburgh, and Gottingen; however, he also eventually graduated from Cambridge University. Before graduation Young received an inheritance from a grand-uncle which was enough to secure his finances for the rest of his life (Fig. 12.1). Young practiced medicine in London but at the same time he became interested in matters connected with light, such as the eye, the origin of the rainbow, and others. He carried out experiments where a beam of light was split in two, and then the rays were brought together again.

### Light as a Wave Phenomenon

What happens when two beams of light are combined? If light is made of particles, the intensity of light should increase: light + light = more light. But if light consists of waves, there is also another possibility: light + light = dark. Imagine waves like those on a lake with high crests above the surface and troughs below. Waves can destroy each other if the trough of one beam's wave strikes the surface at the same time as another wave's opposite phase crest. Young observed this phenomenon which is known as destructive interference (Fig. 12.2). Interference is characteristic of waves. As a very useful application of his experiment, Young was able to determine the tiny distance from one wave crest to the next, the wavelength of light. It varies between 0.4 |im(1 |im = 0.001 mm) for violet light and 0.7 |im for red light.

If light is a wave, what is vibrating? The surface of our lake example vibrates up and down, perpendicular to the direction of motion; it is a transverse wave. A sound wave propagates through air as a compression wave which takes place in

Fig. 12.1 Thomas Young (1773-1829) demonstrated the wave nature of light the direction motion like a compression traveling along a spring. Young showed that light waves are transverse like the lake waves, as was independently found by Augustin Jean Fresnel (1788-1827) a little later. Namely, light can be polarized (as seen in today's familiar Polaroid sun glasses), which is not possible for compression

Fig. 12.2 Young's interference experiment. Light arrives at the screen from two vertical slits. Instead of having two bright lines in the screen, we see a whole lot of alternately bright and dark fringes. At the bright fringes, the waves arriving from the two different slits reenforce each other. The path length difference from the slits to the location is either zero or one whole wavelength, so the two crests match. At the dark fringes, they cancel each other because the path difference is one-half wavelength, so a crest from one matches a trough from the other. The experiment shows that light is a wave phenomenon

Fig. 12.2 Young's interference experiment. Light arrives at the screen from two vertical slits. Instead of having two bright lines in the screen, we see a whole lot of alternately bright and dark fringes. At the bright fringes, the waves arriving from the two different slits reenforce each other. The path length difference from the slits to the location is either zero or one whole wavelength, so the two crests match. At the dark fringes, they cancel each other because the path difference is one-half wavelength, so a crest from one matches a trough from the other. The experiment shows that light is a wave phenomenon

Fig. 12.3 Newton decomposed the sunlight into the colors of the rainbow by using a prism, shown on the right. He then used a second prism like the one on the left to demonstrate that separate colors could not be broken down any more and concluded that light is a heterogenous substance essentially made of different components, colors. Illustration from Newton's Opticks

Fig. 12.3 Newton decomposed the sunlight into the colors of the rainbow by using a prism, shown on the right. He then used a second prism like the one on the left to demonstrate that separate colors could not be broken down any more and concluded that light is a heterogenous substance essentially made of different components, colors. Illustration from Newton's Opticks waves. As an argument against the wave theory, scientists of the time pointed out that the nature of the medium for light waves, the ether, as proposed by Young and Fresnel, was unknown.

As Newton had noted, when a beam of sunlight from a hole passes through a prism, the beam is spread out in all colors of the rainbow which apparently form a continuous band of colors, the spectrum of the Sun (Fig. 12.3). As sketched in the figure, light of a given color could not be broken down further by a second prism. From his clever experiment, Newton concluded that light is a heterogenous substance made of separate components, each having its own color.

Because the different colors of the wide beam overlap, Joseph Fraunhofer (mentioned earlier) used a careful arrangement of a very narrow slit and lenses as well as the prism to avoid overlap of the colors (this device is now called a spectroscope). Examining sunlight, Fraunhofer found that there are apparently missing colors in the spectrum of the Sun! The missing colors show up as dark lines; no image of the narrow slit is formed in those positions or wavelengths of the Sun's spectrum.