Q.38 Physics ⭐ 360° Option Elimination Active

NEET 2025 Physics Question 38 Solution

Topic: Dual Nature of Radiation and Matter · Official NTA Booklet Code 45

Question

Which of the following options represent the variation of photoelectric current with property of light shown on the x-axis?

✓

Verified Against NTA Final Answer Key

Option (1) : A only

📚 Dual Nature of Radiation and Matter Easy

📋 Given Parameters & Problem Setup

Graph A Photoelectric current vs. Intensity: straight line through origin

Graph B Photoelectric current vs. Intensity: horizontal line (independent)

Graph C Photoelectric current vs. Frequency: straight line through origin

Graph D Photoelectric current vs. Frequency: linear increase after threshold frequency

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The 30-Second Exam Hall Shortcut: ⚡ Photoelectric Laws Memory Hook

⏱️ Save ~30 seconds

Photoelectric Current ∝ Intensity ONLY (more photons = more electrons)! Current is INDEPENDENT of frequency (above ν₀)! ⟹ Only Graph A is correct!

1 Law 1: Photoelectric current is directly proportional to the intensity of incident light: I ∝ Intensity. This is a straight line through the origin, which is Graph A.

2 Law 2: Photoelectric current is INDEPENDENT of frequency of incident light (provided frequency ν > ν₀).

3 Notice Graph D: That graph represents Stopping Potential (or K_max) vs. Frequency, NOT Photoelectric Current!

4 Therefore, only Graph A correctly represents the variation of photoelectric current.

5 Select Option (1) "A only" in under 10 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Einstein’s Photoelectric Equation (NCERT Physics · Dual Nature of Radiation and Matter: K_max = hν - Φ).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Analyze Dependence of Photoelectric Current on Intensity

Intensity of light represents the energy incident per unit area per second, which for a given frequency is directly proportional to the number of incident photons per second. Since one photon liberates at most one photoelectron:

I_{photo} \propto \text{Number of photons/sec} \propto \text{Intensity}

• Thus, photoelectric current increases linearly with light intensity, starting from zero at zero intensity.
• Graph A accurately represents this linear proportionality.

Step 2

Analyze Dependence of Photoelectric Current on Frequency

For incident light with frequency ν greater than the threshold frequency ν₀, changing the frequency while maintaining constant intensity does NOT change the number of incident photons (in fact, if energy intensity is constant, photon flux I/(hν) slightly decreases). In standard NCERT experiments where photon flux is maintained:

I_{photo} = \text{constant (independent of frequency for } \nu > \nu_0)

• Photoelectric current does NOT increase linearly with frequency.
• Hence, Graphs C and D are completely incorrect for photoelectric current.

Step 3

Identify the Common Conceptual Confusion in Graph D

Graph D shows a linear relationship with a non-zero threshold intercept:

e V_0 = K_{max} = h\nu - h\nu_0

• This straight line with intercept ν₀ represents Stopping Potential V₀ vs. Frequency ν, NOT Photoelectric Current!
• Therefore, Graph A is the single and only correct representation, corresponding to Option (1).

⭐

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

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Current is directly proportional to intensity (Graph A) and independent of frequency.

💡

Why this is airtight: None; solid conceptual understanding of photoelectric laws.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Photoelectric current does not increase linearly through origin with frequency.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing intensity and frequency dependencies.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Graph D represents stopping potential vs frequency, not current vs frequency!

⚠️

Student Fallacy (Why candidates pick this trap): Failing to read the y-axis label (Photoelectric Current vs Stopping Potential).

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Graph B wrongly claims current is independent of intensity.

⚠️

Student Fallacy (Why candidates pick this trap): Total reversal of photoelectric laws.

⚠️

NTA Trap Alert: Misreading the Y-Axis Label in Graph D

Almost every student recognizes the classic shape of Graph D from NCERT! But in NCERT, Graph D has "Stopping Potential (V₀)" or "Maximum K.E." on the y-axis, NOT "Photoelectric Current". NTA deliberately tested if students read the axes carefully!

🛡️ How to avoid the -1 mark: ALWAYS read the y-axis and x-axis labels before recognizing a familiar graph curve.

📖

Core NEET Syllabus Standard

NEET Physics Core Syllabus · Dual Nature of Radiation and Matter Standard Benchmark

Core Scientific Principle Tested:

"Photoelectric current is directly proportional to the intensity of incident radiation."

Syllabus Focus: Laws of Photoelectric Emission

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Current tracks Photons (Intensity) ⟹ Straight line through origin: Graph A only!

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NEET 2025 Physics Question 38 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Photoelectric Laws Memory Hook

Step-by-Step Rigorous Workflow & Mathematical Proof

Analyze Dependence of Photoelectric Current on Intensity

Analyze Dependence of Photoelectric Current on Frequency

Identify the Common Conceptual Confusion in Graph D

360° Option Elimination Engine Key Feature

NTA Trap Alert: Misreading the Y-Axis Label in Graph D

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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