Introduction

Optics is the branch of physics dealing with light, in exact parallel to acoustics, the branch dealing with sound (already met in the earlier chapter on oscillations and waves). To see anything at all we need light, so understanding what light actually is, and how it behaves, is the natural starting point of this chapter.

Historically, 'seeing' was explained simply as light stimulating the retina of the eye, but the deeper question of what light itself IS took centuries to settle, and is picked up in the very next section. Two connected facts orient the whole chapter: light and sound are both wave phenomena, yet they differ fundamentally in nature (sound needs a material medium to travel through; light does not); and this chapter covers only geometrical (ray) optics -- wave optics (interference, diffraction, polarization) is deliberately deferred to Class XII, where it can be developed properly alongside the wave nature of light introduced here.

A short set of framing questions is worth keeping in mind while reading the chapter, since every one of them is answered fully by its end: what are the laws of reflection and refraction; what is dispersion of light; what is refractive index; what is total internal reflection; how does light refract at a curved surface; and how does a rainbow form.