Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Why is the sign convention used in the derivation of the lens formula and yet used again when it is applied in numerical problems? Won't the whole idea of sign convention be eliminated if it is used twice?

share|cite|improve this question
Can you be more specific about the sign convention, and how it appears twice? It is a bit difficult to understand what you mean here. – Bernhard Apr 7 '13 at 18:10
@GauravNayak, you can get a descriptive answer here too. – user22180 Jul 9 '14 at 21:25

We derive the lens or mirror formula with the sign convention. Then in solving the problem we again use sign convention to de neutralize it.

Also we use sign convention to make the calculation simple on large scale.

To identify the nature of image.

To know where is the image or object is placed with respect to lens or mirror.

share|cite|improve this answer
You could add an example to answer the question better – udiboy1209 Jul 9 '13 at 17:23

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.