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Basic Optical Principle

Ophthalmic lens theory is based upon the basic laws of Geometric Optics.
The ophthalmic lenses are used to alter the path of light passing through it
usually to correct some errors of the eyes. For theoretical calculations on
ophthalmic lenses, light is assumed to travel in straight lines and that the
incident light travels from left to right. It meets a surface that separates
two media. What happens to the light, then, depends upon the nature of
the surface and the two media on either side. In this chapter, we will
consider these fundamental laws of physics and how they relate to the
theory of spectacle lenses.

LAWS OF REFLECTION
Light travels in straight line until it meets a surface that separates the two
media. When light meets the surface, its behavior depends on the nature of
the surface and the two media on either side. Light may be absorbed by the
new medium, or transmitted onwards through it, or it may bounce back
into the first medium. The bouncing back of light into the incident ray
medium is called Reflection of light. The light may reflect specularly if the
surface is acting as true mirror or it may reflect diffusely, scattering it in all
directions if the surface is incompletely polished. In case of specular
reflection, the reflected light obeys the laws of reflection, which states that:

1. The incident ray, the reflected ray and the normal ray to the reflecting
surface – all lie in the same plane.
2. The angle of incidence equals the angle of reflection.
It follows from the laws of reflection that, when an object is placed in front
of a plane mirror, the image formed by the mirror lies as far behind the
mirror surface as the object lies in front. Also the straight line that joins the
object and its reflected image is normal to the mirror surface (Fig. 1.1).