Q.13 Physics ⭐ 360° Option Elimination Active

NEET 2025 Physics Question 13 Solution

Topic: Ray Optics and Optical Instruments · Official NTA Booklet Code 45

Question

In a certain camera, a combination of four similar thin convex lenses are arranged axially in contact. Then the power of the combination and the total magnification in comparison to the power (p) and magnification (m) for each lens will be, respectively

✓

Verified Against NTA Final Answer Key

Option (3) : 4p and m^4

📚 Ray Optics and Optical Instruments Easy

📋 Given Parameters & Problem Setup

Number of Thin Lenses 4 identical thin convex lenses

Arrangement Axially in contact

Single Lens Power & Magnification Power p, Magnification m

⚡

The 30-Second Exam Hall Shortcut: ⚡ Power Adds, Magnification Multiplies

⏱️ Save ~30 seconds

Powers ADD algebraically (P = p + p + p + p = 4p); Magnifications MULTIPLY (M = m · m · m · m = m⁴)!

1 Rule 1 (Power of lenses in contact): Total power is the algebraic sum of individual powers: P = Σ p_i = p + p + p + p = 4p.

2 Rule 2 (Magnification of lens combination): Total magnification is the product of individual magnifications: M = Π m_i = m × m × m × m = m⁴.

3 Result: Power = 4p, Magnification = m⁴.

4 Match Option (3) instantly!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Physics · Ray Optics (Prism Formula at Minimum Deviation: μ = sin((A+δ_m)/2) / sin(A/2)).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Power of Thin Lenses in Contact

For n thin lenses placed axially in contact, the equivalent focal length F satisfies 1/F = 1/f₁ + 1/f₂ + ... + 1/f_n. Since power P = 1/f:

P_{eq} = p_1 + p_2 + p_3 + p_4 = p + p + p + p = 4p

• Power is strictly additive for lenses in contact.

Step 2

Total Magnification of Lens System

The image formed by the first lens acts as the object for the second lens, and so forth. Consequently, the total lateral magnification produced by the lens system is the product of the magnifications produced by each individual lens:

M_{eq} = m_1 \times m_2 \times m_3 \times m_4 = m \times m \times m \times m = m^4

• Thus, the combination has power 4p and magnification m⁴, corresponding to Option (3).

⭐

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 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Magnifications multiply, they do not add (m⁴, not 4m).

⚠️

Student Fallacy (Why candidates pick this trap): Treating magnification as an additive property.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Both power (should be additive 4p) and magnification (should be multiplicative m⁴) are wrong.

⚠️

Student Fallacy (Why candidates pick this trap): Swapping operations completely.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Powers add (4p), magnifications multiply (m⁴).

💡

Why this is airtight: None; canonical optics laws.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Powers add, they do not multiply.

⚠️

Student Fallacy (Why candidates pick this trap): Multiplying powers instead of adding them.

⚠️

NTA Trap Alert: Add vs. Multiply Operation Confusion

Students frequently mix up which property adds and which multiplies. Power adds (P = Σ p_i); Magnification multiplies (M = Π m_i).

🛡️ How to avoid the -1 mark: Mnemonic: "P adds, M multiplies" (PAM).

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Core NEET Syllabus Standard

NEET Physics Core Syllabus · Ray Optics and Optical Instruments Standard Benchmark

Core Scientific Principle Tested:

"P = p₁ + p₂ + ... + p_n and m = m₁ × m₂ × ... × m_n."

Syllabus Focus: Combination of Thin Lenses in Contact

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Powers PLUS, Magnifications MULTIPLY: 4p and m⁴!

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NEET 2025 Physics Question 13 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Power Adds, Magnification Multiplies

Step-by-Step Rigorous Workflow & Mathematical Proof

Power of Thin Lenses in Contact

Total Magnification of Lens System

360° Option Elimination Engine Key Feature

NTA Trap Alert: Add vs. Multiply Operation Confusion

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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