A ray diagram for a convex lens is shown. Which ray rule is illustrated by the undeviated central ray? 
AA ray parallel to the principal axis passes through the second focus after refraction
BA ray through the first focus emerges parallel to the principal axis
CA ray passing through the optical centre emerges without deviation
DA ray striking the lens normally is totally internally reflected
Answer & Solution
Correct answer: C. A ray passing through the optical centre emerges without deviation
In thin-lens ray tracing, one of the standard construction rules is that a ray passing through the optical centre emerges undeviated. That is the straight central ray shown in the convex-lens diagram. The other listed ray rules are valid in other situations, but they do not describe the undeviated central ray.
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