Which statement correctly describes the principal foci of a thin lens?  
AA lens has only one focus, located on the image side
BThe two foci of a double convex or double concave lens are equidistant from the optical centre
CThe first focal point always lies farther from the lens than the second focal point
DA concave lens has no real geometric focus on either side
Answer & Solution
Correct answer: B. The two foci of a double convex or double concave lens are equidistant from the optical centre
A thin lens has two principal foci, one on each side, and for a symmetric thin-lens treatment they are equidistant from the optical centre. The focus on the side of the original light source is the first focal point; the other is the second focal point. So B is the correct statement.
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