Arrange the following steps (A, B, C and D) in proper sequence to determine the index correction in the given arrangement.
(A) The difference between the two gives the correction to be applied to the observed distance.
(B) The positions of the uprights are read on the scale. Their difference gives the observed distance between tip of the pin and the pole of the mirror.
(C) A thin straight index needle OP is placed so that its one end O touches the tip of the pin and the other end P touches the pole of the mirror.
(D) Length of the needle OP is measured by placing it on the scale which is the actual distance between the points in question.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B), (C), (D), (A)
The index correction is the difference between the actual distance between the pin tip and the pole of the mirror and the observed distance read from the uprights on the optical bench scale. It is applied to every distance read later on the bench.
(B) First the positions of the two uprights are read on the scale, with the pin and the mirror as set up. Their difference is the observed distance \( y \).
(C) Next the index needle OP is fitted between the tip of the pin and the pole of the mirror.
(D) The needle is then taken out and its length measured on the scale. This is the actual distance \( x \).
(A) Only now are both distances known, so their difference \( x - y \), the index correction, comes last.
The order is B, C, D, A.