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Basic Optical Principle 3

Fig. 1.2: Refraction of light entering an optical dense medium from air
VERGENCE

Vergence is the term that determines the direction and power of light
transmission. It is the wavefront of light of a particular position in a
particular time. The vergence of light is defined by:
n
V =
L
Where n = refractive index of the material.
L = the distance in accordance with the Cartesian Sign
Convention.
Since the distance L is measured from the wavefront and the light is
1
traveling from left to right, it is a negative distance and the vergence is
negative (divergent). L is positive since it is directed to the right from the
2
wavefront (convergent) (Fig. 1.3). Parallel light rays are said to have zero
vergence. The unit to measure vergence is “dioptre”. The change in vergence
when the light encounters a refracting surface is equal to the power of the
surface.

Fig. 1.3: Vergence is measured from the wavefront.
Positive is in the direction of light travel