![]() Young used sunlight, where each wavelength forms its own pattern, making the effect more difficult to see. Also, because S 1 S 1 and S 2 S 2 are the same distance from S 0 S 0, the amplitudes of the two Huygens wavelets are equal. Two independent light sources (which may be two separate areas within the same lamp or the Sun) would generally not emit their light in unison, that is, not coherently. The term incoherent means the waves have random phase relationships, which would be the case if S 1 S 1 and S 2 S 2 were illuminated by two independent light sources, rather than a single source S 0 S 0. By coherent waves, we mean the waves are in phase or have a definite phase relationship. The sources S 1 S 1 and S 2 S 2 are then said to be coherent. Since S 0 S 0 is assumed to be a point source of monochromatic light, the secondary Huygens wavelets leaving S 1 S 1 and S 2 S 2 always maintain a constant phase difference (zero in this case because S 1 S 1 and S 2 S 2 are equidistant from S 0 S 0) and have the same frequency. Fringes produced by interfering Huygens wavelets from slits S 1 S 1 and S 2 S 2 are observed on the screen. This pattern, called fringes, can only be explained through interference, a wave phenomenon.įigure 3.3 The double-slit interference experiment using monochromatic light and narrow slits. The light emanating from the two pinholes then fell on a screen where a pattern of bright and dark spots was observed. ![]() The emerging beam fell on two pinholes on a second board. In Young’s experiment, sunlight was passed through a pinhole on a board. If light is an electromagnetic wave, it must therefore exhibit interference effects under appropriate circumstances. If there were not one but two sources of waves, the waves could be made to interfere, as in the case of waves on water ( Figure 3.2). The acceptance of the wave character of light came many years later in 1801, when the English physicist and physician Thomas Young (1773–1829) demonstrated optical interference with his now-classic double-slit experiment. Owing to Newton’s tremendous reputation, his view generally prevailed the fact that Huygens’s principle worked was not considered direct evidence proving that light is a wave. Newton thought that there were other explanations for color, and for the interference and diffraction effects that were observable at the time. ![]() The Dutch physicist Christiaan Huygens (1629–1695) thought that light was a wave, but Isaac Newton did not. Define constructive and destructive interference for a double slit.By the end of this section, you will be able to: ![]()
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