A camera consists essentially of two parts: a detector and a lens construction. The detector is some surface that can record the light which hits it. Old-school cameras used the chemical reaction of silver-oxidation under light, whereas modern cameras use electronic photo-detectors.

While the detector is important, that which really makes or brakes a camera is the lens. The lens' job is to take the light reflected off some object (that which you are taking a picture of) and redirect it in an optimal way so that a faithful image forms on the detection surface. The image has to form exactly at the right distance $d_i$ (so that it is in focus) and have exactly the right height $h_i$ (so it fits on the detector).

To understand how lenses transform light, there is just one equation you need to know: \[ \frac{1}{d_o} + \frac{1}{d_i} = \frac{1}{f}, \] where $d_o$ is the distance from the object to the lens, $d_i$ is the distance from the lens to the image and $f$ is called the focal length of the lens. This entire chapter is dedicated to this equation and its applications. It turns out that curved mirrors behave very similarly to lenses, and the same equation can be used to calculate the properties of the images formed by mirrors. Before we talk about curved mirrors and lenses, we will have to learn about the basic properties of light and the laws of reflection and refraction.