2. Offsets are measured with an accuracy of 1 in 40. If the point on the paper from both sources of error (due to angular and measurement errors) is not to exceed 0.05 cm on a scale of 1 cm = 20 m, the maximum length of offset should be limited to
4. 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