Introduction

WBCHSE's Unit 6, "Optics", is built in two sub-topics. SUB TOPIC 1 treats light within the ray (geometrical) picture -- reflection and refraction at plane and curved surfaces, lenses, mirrors, and the optical instruments built from them -- where light is followed as straight-line rays and a wavefront never explicitly appears. This second sub-topic, Wave Optics, asks a different question: what is light, physically, that makes those ray laws work at all? The answer developed here is Huygens' wave picture, in which light is treated as an advancing wavefront, and the familiar laws of reflection and refraction are derived afresh from that wavefront picture rather than assumed. The chapter then turns to phenomena the ray picture cannot explain at all -- interference, seen as the alternating bright and dark fringes produced when two coherent beams of light from Young's double slit overlap, and diffraction, the spreading of light into the geometrical shadow region at a narrow single slit -- both of which are decisive evidence that light truly behaves as a wave.

Diffraction, in turn, sets a hard physical limit on how finely any optical instrument -- a microscope, an astronomical telescope -- can distinguish two closely spaced objects, a limit expressed through the instrument's resolving power. The chapter closes with polarization, the confinement of a light wave's vibrations to a single plane, which is possible only because light is a transverse wave, and with Brewster's law and the everyday uses of polaroid material that follow from it.