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Surveying - Engineering

Q1:

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.

ANS:C -

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).