# Mirrors and Lenses

## Introduction

In another moment Alice was through the glass, and had jumped lightly down into the Looking-glass room.
—Through the Looking Glass by Lewis Carol
Through the Looking Glass tells of the adventures of Alice after she steps from the real world, through a mirror, and into the virtual world. In this chapter we examine the optical meanings of real and virtual, as well as other concepts that make up the field of optics.
The light from this page or screen is formed into an image by the lens of your eyes, much as the lens of the camera that made the photograph at the beginning of this chapter. Mirrors, like lenses, can also form images, which in turn are captured by your eyes.
Optics is the branch of physics that deals with the behavior of visible light and other electromagnetic waves. For now, we concentrate on the propagation of light and its interaction with matter.
It is convenient to divide optics into two major parts based on the size of objects that light encounters. When light interacts with an object that is several times as large as the light’s wavelength, its observable behavior is similar to a ray;  it does not display its wave characteristics prominently. We call this part of optics geometric optics. This chapter focuses on situations for which geometric optics is suited.