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Basic Optical Principle 7
SIGN CONVENTION
Fig. 1.8: Sign convention
Light is assumed to be passing from left to right and the measurement
taken in the course of light is taken as positive and the measurement taken
against the course of light is taken as negative. All distances are measured
from the vertex of the lens to the point in question. The sign convention is
used to represent whether the corresponding surface is convex or concave
(Fig. 1.8). It says that:
• Distances to the left are negative
• Distances to the right are positive
• Distances above the optical axis are positive
• Distances below the optical axis are negative.
