TABLE OF CONTENTS In 1801, Thomas Young performed an experiment that lead to the establishment of the wave theory of light. His arrangement consists of two small, closely spaced holes or slits, S 1 and S 2, in an opaque surface with a light source S placed behind it, as shown in Figure 2. Here S 1 and S 2 serve as two coherent sources of light of the same intensity. Young used sunlight as a light source and observed an interference pattern on a screen placed parallel to the two sources S 1 and S 2. A series of a few colored bright and dark vertical lines called fringes appear on the screen. As a modification of the original experimental arrangement, sunlight is replaced by monochromatic light and then we get alternatively dark and bright bands. Let us draw circles of radii ?/2, 2 ?/2, 3 ?/2, ..., n ?/2 around S 1 and S 2; the alternate circles represent crests and troughs.
