Introduction

WBCHSE's Semester IV Unit 6, "Optics", is split into two sub-topics. This first sub-topic, Ray Optics and Optical Instruments, studies light as a collection of straight-line rays -- the geometrical-optics picture that is valid whenever the obstacles and openings light meets are large compared with its wavelength, so that diffraction effects can be ignored. Within this ray picture the chapter builds up, in order, the three basic ray-bending events light can undergo at a smooth surface -- reflection at a plane or spherical mirror, refraction at a plane or spherical boundary between two transparent media, and the special case of refraction called total internal reflection -- and then combines these building blocks into the working principle of every important optical device: lenses (single and in combination, and combined with mirrors), the prism, and the optical instruments built from lenses and mirrors, namely the human eye itself, the simple and compound microscope, and the refracting and reflecting astronomical telescope. The second sub-topic, Wave Optics, later asks what happens when the wave nature of light cannot be ignored (interference, diffraction, polarisation); everything in this chapter deliberately stays within the simpler ray (particle-trajectory) approximation, which is entirely sufficient to explain how mirrors, lenses, prisms and every instrument built from them actually form images.