Consider the following statements about the early observations on photoelectric emission:
(A) Heinrich Hertz discovered photoelectric emission in 1887, during his electromagnetic wave experiments.
(B) Lenard found that the current in his tube stopped as soon as the ultraviolet radiation was stopped.
(C) Hallwachs found that a negatively charged zinc plate gained more negative charge when ultraviolet light fell on it.
(D) Hallwachs and Lenard found that the threshold frequency did not depend on the material of the emitter plate.
(E) Alkali metals such as lithium, sodium and potassium were found to be sensitive even to visible light.
Which of the above statements are incorrect?
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C) and (D) only
Hertz discovered the effect. Hallwachs and Lenard studied it in detail. In their experiments, ultraviolet light made metal plates give out negatively charged particles.
(A) Heinrich Hertz discovered photoelectric emission in 1887, during his electromagnetic wave experiments. — Correct statement. Hertz saw that ultraviolet light from an arc lamp made the sparks across his detector loop stronger.
(B) Lenard found that the current in his tube stopped as soon as the ultraviolet radiation was stopped. — Correct statement. The light falling on the emitter caused the current, so the current ended with the light.
(C) Hallwachs found that a negatively charged zinc plate gained more negative charge when ultraviolet light fell on it. — Incorrect statement. The negatively charged plate lost its charge, because negatively charged particles left it under ultraviolet light.
(D) Hallwachs and Lenard found that the threshold frequency did not depend on the material of the emitter plate. — Incorrect statement. The threshold frequency depends on the material of the emitter plate, so it differs from metal to metal.
(E) Alkali metals such as lithium, sodium and potassium were found to be sensitive even to visible light. — Correct statement. Zinc, cadmium and magnesium needed ultraviolet light, but these alkali metals gave out electrons with visible light too.
NCERT (2026-27):
Hertz’s observations – “The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz (1857-1894), during his electromagnetic wave experiments. …”
Hallwachs’ and Lenard’s observations – “… As soon as the ultraviolet radiations were stopped, the current flow also stopped. …”
Hallwachs’ and Lenard’s observations – “Hallwachs, in 1888, undertook the study further and connected a negatively charged zinc plate to an electroscope. He observed that the zinc plate lost its charge when it was illuminated by ultraviolet light. …”
Hallwachs’ and Lenard’s observations – “… This minimum frequency depends on the nature of the material of the emitter plate.”
Hallwachs’ and Lenard’s observations – “… However, some alkali metals such as lithium, sodium, potassium, caesium and rubidium were sensitive even to visible light. …”