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Physics Question 20 – NEET-UG 2026

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Which of the following measurements require 'index correction' ?

Index correction refers to an adjustment made to a measured value to account for a systematic error or a deviation from an ideal condition in an experiment.

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Ninja Strategy•Recall Standard Experiments

Recognize that 'index correction' (or end correction) is a well-known and specific requirement for the resonance tube experiment due to the antinode forming outside the physical end of the tube.

Step 1: Define Index Correction✦ Active

Index correction (or end correction) is a specific type of systematic error correction applied in certain physics experiments where the effective length or position of a vibrating system differs from the directly measured physical length. This is often due to the boundary conditions not being perfectly at the physical end of the apparatus.

Step 2: Analyze Each Option for Index Correction Requirement○ Expand

(1) Measurement of resistance using a meter bridge: This involves measuring lengths on a wire. While general scale errors might exist, a specific 'index correction' due to an inherent physical offset is not typically applied. (2) Measurement of gravitational acceleration using a simple pendulum: This involves measuring the length of the pendulum and its time period. Length measurement might have zero error, but it's not an 'index correction' specific to the pendulum's oscillation physics. (3) Measurement of focal length using an optical bench: This involves measuring distances between lenses/mirrors and objects/images. Similar to the above, general scale errors might occur, but not a specific 'index correction' inherent to the optical setup. (4) Measurement of speed of sound using a resonance tube: In this experiment, the antinode of the standing wave formed in the air column does not occur exactly at the open end of the tube. It forms slightly outside the open end. This small additional length is known as 'end correction' or 'index correction', and it must be added to the measured length of the air column to get the effective length for resonance calculations. For example, if L1 and L2 are the first and second resonant lengths, the effective lengths are L1+e and L2+e, where e is the end correction.

💡 Teacher's Secret Hint

Remember that end correction is crucial for accurate calculations of wavelength and speed of sound in resonance tube experiments.

Step 3: Conclusion○ Expand

Based on the analysis, the measurement of the speed of sound using a resonance tube is the experiment that specifically requires 'index correction' (end correction).

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