Introduction

This chapter is a brief introduction to the paraxial theory of reflecting optics. The term “mirror” has not been used because although all refracting surfaces also act as reflecting surfaces, they cannot be classified as mirrors. Here mirrors are defined as reflecting surfaces where there is no transmission of rays.

Although reflecting optics do not have a very large role to play in the optics of the eye or visual and ophthalmic instruments, they are very useful and important in some cases. The reflections from the four refracting surfaces of the eye are most useful as they can be used to measure the radii of curvature of these surfaces. Measurement of the radius of curvature of the cornea is a special case and is called keratometry. Reflections from the refracting surfaces of the eye are known as Purkinje images.

Many optical situations involving reflections also involve some refraction. For example in the measurement of the radius of curvature of the front surface of the crystalline lens of the eye, the beam is refracted by the cornea, reflected from the front surface of the lens and then refracted by the cornea once again. Optical systems that are a mix of refracting and reflecting elements are called catadioptric systems (see Section 4.2). Those that are purely reflecting are called catoptric. However, very few optical systems are catoptric.

Reflecting components are often used instead of refracting components because they can be made with a smaller mass and they have no chromatic aberration. They are also useful with high energy beams, where the smallest amount of absorption would damage a lens.