Which of the following measurements require ‘index correction’ ?

In an optical bench experiment, the distance between the index marks on the uprights may not be exactly equal to the actual distance between the optical centre of the lens and the tip of the pin. This difference is known as index error.

To obtain the true distance, an index correction must be applied to the observed distance. Therefore, the measurement of the focal length of lenses using an optical bench requires index correction.

In contrast, the resonance tube and meter bridge experiments require 'end correction'.

Answer: Measurement of focal length of lenses using optical bench