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QPage 363: Q10: HC Verma: Concepts of Physics - Volume 2 Photoelectric Effect and Wave-Particle Duality Page 363: Q10 Solution: The threshold wavelength of a metal is . Light of wavelength…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often ignore that an isolated metal losing electrons becomes positively charged, creating a retarding potential that reduces the effective kinetic energy of newly emitted electrons. Even if the photon energy ($h\nu$) is slightly above the work function, the growing positive charge can raise the stopping potential until $h\nu$ no longer exceeds $\phi+eV_{ret}$, stopping emission.

The threshold wavelength of a metal is λ0\lambda_0. Light of wavelength slightly less than λ0\lambda_0 is incident on an insulated plate made of this metal. It is found that photoelectrons are emitted for some time and after that the emission stops. Explain.

Step-by-Step Verified Solution

**Answer:** The photoelectrons are emitted for some time and then the emission stops because the metal plate becomes positively charged. **Step-by-Step Explanation:** The metal plate is insulated. As photoelectrons are emitted, the plate loses negative charge and becomes positively charged. This positive charge attracts the emitted electrons back, creating a retarding potential. When this potential becomes equal to the stopping potential, emission stops.

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