# Civil Engineering - Surveying - Discussion

### Discussion :: Surveying - Section 1 (Q.No.12)

12.

If f1 and f2 are the distances from the optical centre of a convex lens of focal length f to conjugate two points P1 and P2 respectively, the following relationship holds good

 [A]. f = f1 + f2 [B]. f = (f1 + f2) [C]. [D]. none of these.

Explanation:

No answer description available for this question.

 Nayila Salim Almanji said: (Feb 11, 2017) I want the description of this answer. Please explain it.

 Rani said: (Feb 12, 2018) In tachometry focal length i.e, the length of focus f (inverse) is equal to the inverse length of (i) length between the lens and intercept staff f1 (ii) length between crosshair and lens f2. 1/f=(1/f1)+(1/f2).

 Rabindra said: (May 1, 2018) Nice explanation, thanks @Rani.

 V.V.Savaliya said: (Oct 11, 2018) Thanks @Rani.

 Aakash said: (Oct 30, 2018) Thanks.

 Jay said: (Jan 22, 2019) Thanks @Rani.

 Muhammad Umar said: (Feb 1, 2020) Well explained @Rani.

 Kanaka Raju said: (Sep 21, 2020) Thanks for the explanation @Rani.

 Kanaka Raju said: (Sep 21, 2020) Thanks all.