Introduction

What this section does

Section 7.1 is a recap, not new theory. It reminds you what ray optics (geometrical optics) already assumes — that light travels in straight lines through a uniform homogeneous medium, and that the straight-line path is called a ray — and restates the laws of reflection and refraction that you will now be asked to DERIVE from the wave picture in Sections 7.5 and 7.6.

The laws you are expected to already know

• Laws of reflection: (i) the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane; (ii) the angle of incidence equals the angle of reflection, $\angle i=\angle r$.
• Laws of refraction: (i) the incident ray, the refracted ray and the normal at the point of incidence lie in the same plane; (ii) $n_{1}\sin i=n_{2}\sin r$, where $n_{1}$ and $n_{2}$ are the absolute refractive indices of the two media.
• The position and nature of an image formed by a mirror or lens depend on the object position and on the focal length.

Why wave optics is needed at all

Geometrical optics works for shadows, mirrors and lenses only because the wavelength of light is very much smaller than the mirrors, lenses and obstacles we meet in the laboratory. When the obstacle or aperture becomes comparable with the wavelength, the ray picture fails and the wave picture — wave optics — takes over.