Q.8 Physics ⭐ 360° Option Elimination Active

NEET 2026 Physics Question 8 Solution

Topic: Dual Nature of Matter & Radiation · Official NTA Booklet Code 50

Question

A beam of light falls on a metal surface such that photo-electrons are generated. If power of the light source starts to decrease linearly with time t, then variation of the photocurrent I and magnitude of the stopping potential |V| with time is best represented by:

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Verified Against NTA Final Answer Key

Option (1) : Photocurrent I decreases linearly with time; Stopping potential |V| remains constant

📚 Dual Nature of Matter & Radiation Easy

📋 Given Parameters & Problem Setup

Incident Light Power P(t) Decreases linearly with time t

Light Frequency nu Constant (photoelectrons are generated, nu > nu_0)

Work Function Phi Constant material property of the metal

Variables to Plot Photocurrent I vs t and Stopping Potential |V| vs t

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The 30-Second Exam Hall Shortcut: ⚡ Einstein Photoelectric Decoupling Rule

⏱️ Instant conceptual recognition saves 1 full minute.

Intensity controls current; Frequency controls stopping potential!

1 Photocurrent I depends directly on the number of photons per second, which is proportional to light Power P(t).

2 Since P(t) decreases linearly with time, Photocurrent I MUST decrease linearly with time (straight line with negative slope).

3 Stopping potential |V_0| = (h*nu - Phi) / e depends ONLY on frequency nu and work function Phi. Power does not enter this equation!

4 Since frequency is unchanged, Stopping Potential |V| remains completely CONSTANT (horizontal flat line).

5 Look at the graphs: Graph (1) shows I decreasing linearly and |V| horizontal constant. Solved in 10 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Einstein's Photoelectric Equation: e*|V_0| = K_max = h*nu - Phi. Since neither h, nu, nor Phi change with power, |V_0| is strictly time-invariant.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Analyze Photocurrent Dependency on Light Power

The number of incident photons per second is given by n = P(t) / (h*nu). Since each absorbed photon can eject at most one photoelectron (with quantum efficiency eta):

I = \eta e \left( \frac{P(t)}{h\nu} \right) \propto P(t)

• Because power P(t) decreases linearly with time (P(t) = P_0 - k*t), photocurrent I must also decrease linearly with time.
• The graph of I vs t is a straight line sloping downward to zero.

Step 2

Analyze Stopping Potential Dependency on Light Power

Apply Einstein's photoelectric equation for maximum kinetic energy of emitted electrons:

K_{\text{max}} = h\nu - \Phi = e |V_0| \implies |V_0| = \frac{h\nu - \Phi}{e}

• Stopping potential depends solely on the frequency nu of incident radiation and the work function Phi.
• Stopping potential is completely INDEPENDENT of light intensity and source power.
• Therefore, |V| vs t is a horizontal straight line with zero slope.

Step 3

Match with the Given Graph Options

Inspect the four graph sets in the question crop:

\text{Set (1): } I(t) \text{ linear decrease, } |V|(t) \text{ horizontal constant line}

• Matches Set (1) perfectly, corresponding to Option (1).

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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:

Scientifically exact application of Einstein's photoelectric theory; confirmed by Official NTA Key.

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Why this is airtight: Correctly decouples intensity (photon count) from frequency (photon energy).

Note: Flawless physics.

2 Option (2)

✗ Incorrect

Verdict & Scientific Reason:

Classical wave theory error.

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Student Fallacy (Why candidates pick this trap): Believes higher power/intensity gives electrons more energy, so reducing power reduces stopping potential.

Note: This was the exact failure of classical physics that Einstein resolved in 1905.

3 Option (3)

✗ Incorrect

Verdict & Scientific Reason:

Inverted dependencies.

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Student Fallacy (Why candidates pick this trap): Confuses current and stopping potential dependencies.

Note: Current is proportional to power, never constant when power drops.

4 Option (4)

✗ Incorrect

Verdict & Scientific Reason:

Ignores the explicit decrease in source power.

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Student Fallacy (Why candidates pick this trap): Fails to connect power to photon arrival rate.

Note: Fewer photons per second directly reduces photoelectrons ejected.

⚠️

NTA Trap Alert: The Classical Intensity-Energy Misconception

Students who do not master the quantum theory of light assume stopping potential decreases when light gets dimmer, incorrectly choosing Graph (2).

🛡️ How to avoid the -1 mark: Remember the golden rule: Intensity changes NUMBER of electrons (Current). Frequency changes ENERGY of electrons (Stopping Potential).

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Core NEET Syllabus Standard

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

Core Scientific Principle Tested:

"Stopping potential is determined strictly by incident photon frequency: e*V_0 = h*nu - Phi, independent of intensity/power."

Syllabus Focus: Photoelectric Effect, Einstein Equation, and Stopping Potential

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2026 Physics Question 8 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Einstein Photoelectric Decoupling Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Analyze Photocurrent Dependency on Light Power

Analyze Stopping Potential Dependency on Light Power

Match with the Given Graph Options

360° Option Elimination Engine Key Feature

NTA Trap Alert: The Classical Intensity-Energy Misconception

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Dual Nature of Radiation and Matter

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Ratio of de Broglie Wavelength of Photon to Electron with Equal Energy

Dependence of Photoelectric Current on Intensity and Frequency of Light

Characteristics and Properties of Photons

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