Question

According to sign convention, which of the distances measured in the figure are considered to be negative?

• A. a, b
• B. c, b
• C. a, d
• D. c, d

Answer 1

The correct option is B c, b

Hey, we just learnt this! Let's recall. All distances are measured from the pole, P. as also seen in the figure.

Now, distances measured along the principal axis are considered to be positive if along the direction of incident rays, and negative in the opposite direction. In this question, the incident rays are moving towards right, so, a is positive, whereas, c is negative.
Further, for the distances perpendicular to the principal axis, the distance above the principal axis are positive and below are negative. So, d is positive and b is negative. Hence, c and b are negative.