Q.26 Physics ⭐ 360° Option Elimination Active

NEET 2026 Physics Question 26 Solution

Topic: Units & Measurements / Experimental Skills · Official NTA Booklet Code 50

Question

One main scale division of a Vernier calliper is equal to 1 mm and the number of divisions on the Vernier scale is 10. When both the jaws touch each other, the Vernier scale shifts to the left of zero of the main scale in such a way that 4th Vernier division coincides with a division of the main scale. If this Vernier calliper measures the length of a wire to be 1 cm, the actual length of the wire is:

✓

Verified Against NTA Final Answer Key

Option (1) : Awarded Full Marks to All Candidates (Official NTA Key: A,B,C,D)

📚 Units & Measurements / Experimental Skills Challenging

📋 Given Parameters & Problem Setup

1 Main Scale Division (MSD) 1 mm = 0.1 cm

Total Vernier Scale Divisions (VSD) N = 10 divisions

Least Count (assumed standard) LC = 1 MSD - 9/10 MSD = 0.1 mm = 0.01 cm

Zero Error Condition Vernier shifts to left of main scale zero; 4th VSD coincides with a main scale division

Measured Reading 1.0 cm

Official NTA Decision Bonus / All Options Correct (A,B,C,D) due to option-specification ambiguity

⚡

The 30-Second Exam Hall Shortcut: ⚡ The NTA Bonus Diagnostic (Why All Options Were Awarded Marks)

⏱️ Essential exam-hall awareness of NTA challenge resolution.

Understand why this question was awarded full marks by NTA!

1 Standard NCERT negative zero error formula: Zero Error = -(Total Divisions - Coinciding Division) * LC = -(10 - 4) * 0.1 mm = -0.6 mm = -0.06 cm.

2 Corrected Reading = Observed Reading - Zero Error = 1.0 cm - (-0.06 cm) = 1.06 cm.

3 Notice: 1.06 cm is NOT listed in ANY of the four options (0.60 cm, 0.96 cm, 1.00 cm, 1.04 cm)!

4 If one carelessly used the incorrect formula: Zero Error = -4 * LC = -0.04 cm ==> Reading = 1.0 - (-0.04) = 1.04 cm (Option 4).

5 Or if one subtracted: 1.0 - 0.04 = 0.96 cm (Option 2).

6 Because the true NCERT answer (1.06 cm) was missing from the paper, NTA challenged and declared BONUS (A, B, C, D)!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Laboratory Manual explicitly defines negative zero error as: Zero error = -(N - n) * LC, where N is the total number of Vernier divisions and n is the coinciding division. Since N=10 and n=4, error is -0.6 mm, yielding 1.06 cm.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate the Least Count (LC) of the Instrument

For a standard Vernier calliper where 10 Vernier scale divisions equal 9 main scale divisions:

\text{LC} = 1\text{ MSD} - 1\text{ VSD} = 1\text{ mm} - \frac{9}{10}(1\text{ mm}) = 0.1\text{ mm} = 0.01\text{ cm}

• 1 MSD = 1 mm.
• LC = 0.1 mm = 0.01 cm.

Step 2

Apply the Canonical NCERT Negative Zero Error Formula

When the Vernier zero lies to the LEFT of the main scale zero, the error is negative. By NCERT convention:

e = -(N - n) \times \text{LC} = -(10 - 4) \times 0.1\text{ mm} = -0.6\text{ mm} = -0.06\text{ cm}

• N = 10 (total divisions on Vernier scale).
• n = 4 (coinciding division).
• True zero error is -0.06 cm.

Step 3

Compute Actual Length & Identify Option Discrepancy

Actual length equals observed reading minus zero error:

L_{\text{actual}} = L_{\text{observed}} - e = 1.0\text{ cm} - (-0.06\text{ cm}) = 1.06\text{ cm}

• However, the options printed by NTA were: (1) 0.60 cm, (2) 0.96 cm, (3) 1.00 cm, (4) 1.04 cm.
• The correct scientific answer 1.06 cm was missing because the question setter erroneously calculated error as -4 * LC = -0.04 cm (giving Option 4) or subtracted it (giving Option 2).
• During answer key challenge review, NTA accepted the flaw and awarded FULL MARKS to all candidates (Key: A,B,C,D).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 Option (1)

✓ 100% Correct

Verdict & Scientific Reason:

Awarded full marks by NTA (Official Final Key: A,B,C,D).

💡

Why this is airtight: Represents 6 * LC = 0.60 cm confused with final length.

Note: Accepted as correct under official NTA bonus declaration.

2 Option (2)

✓ 100% Correct

Verdict & Scientific Reason:

Awarded full marks by NTA (Official Final Key: A,B,C,D).

💡

Why this is airtight: Results from subtracting 0.04 cm from 1.00 cm (confusing negative error addition with subtraction).

Note: Accepted as correct under official NTA bonus declaration.

3 Option (3)

✓ 100% Correct

Verdict & Scientific Reason:

Awarded full marks by NTA (Official Final Key: A,B,C,D).

💡

Why this is airtight: Ignores zero error entirely.

Note: Accepted as correct under official NTA bonus declaration.

4 Option (4)

✓ 100% Correct

Verdict & Scientific Reason:

Awarded full marks by NTA (Official Final Key: A,B,C,D).

💡

Why this is airtight: The question setter's original intended answer using flawed zero error -0.04 cm.

Note: Accepted as correct under official NTA bonus declaration.

⚠️

NTA Trap Alert: The Negative Zero Error (N - n) Formula Trap

Positive zero error is +n * LC. But NEGATIVE zero error is -(N - n) * LC! Setting e = -n * LC is the classic mistake that caused this entire question to become a bonus.

🛡️ How to avoid the -1 mark: For negative zero error, always count backward from the 10th division: (10 - coinciding division) * LC.

📖

Core NEET Syllabus Standard

NEET UG Physics Core Standards · Experimental Skills & Practical Physics Standard Benchmark

Core Scientific Principle Tested:

"Zero error correction: True Reading = Observed Reading - (Zero Error). Negative zero error = -(N - n) * LC."

Syllabus Focus: Vernier Callipers: Principle, Least Count, and Zero Error Corrections

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2026 Physics Question 26 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The NTA Bonus Diagnostic (Why All Options Were Awarded Marks)

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate the Least Count (LC) of the Instrument

Apply the Canonical NCERT Negative Zero Error Formula

Compute Actual Length & Identify Option Discrepancy

360° Option Elimination Engine Key Feature

NTA Trap Alert: The Negative Zero Error (N - n) Formula Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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