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2 Ophthalmic Lenses

Fig. 1.1: Reflection at the plane surface

LAWS OF REFRACTION

Refraction is defined as the change in direction of light when it passes from
one transparent medium into another of different optical density. The
incident ray, the refracted ray and the normal – all lies in the same plane.
The refracted light undergoes a change in velocity. The ratio of the velocities
of light in the first and second media is called the relative refractive index
between the media, which slows with denser medium. The absolute
refractive index of a medium “n” is defined as the ratio of the velocity of
light in vacuum to the velocity of light in the respective medium:

Velocity of light in vacuum
Refractive Index (n) =
Velocity of light in the medium
The Figure 1.2 illustrates a ray of light that is incident, in air at point D
on the surface of a plane – solid glass block. The refractive index of the first
medium (air) is denoted by n and the refractive index of the second medium
(glass block) is denoted by n’. The angle of incidence that the incident ray
makes with the normal to the surface at D is denoted by ‘n’ and the angle
of refraction is denoted by n’. The laws of refraction (Snell’s law) state that:
1. The incident ray, the refracted ray and the normal to the surface at the
point of incidence – all lie in one plane.
2. The ratio of the angle of incidence, i, to the angle of refraction, i’, is
constant for any two media.
The most important application of this Snell’s law of refraction is to
determine the effect of prisms and lenses on the incident light, that is to
determine the change in direction and the change in vergence produced
by a lens.