Skip to content
GK24
Prime
General ScienceMedium

The photoelectric effect was first observed by which scientist?

  1. A.Albert Einstein
  2. B.Heinrich Hertz
  3. C.Max Planck
  4. D.Robert Millikan

Correct answer

B. Heinrich Hertz

Explanation

The correct answer is B, Heinrich Hertz. In 1887, while studying the sparks produced across a gap between two electrodes, Hertz noticed that the spark passed more easily when ultraviolet light fell on the electrodes, which was the first observation of the photoelectric effect; Hallwachs and Lenard then studied it systematically. Option A is wrong because Einstein did not observe the effect but explained it in 1905 using the idea of light quanta, for which he received the Nobel Prize for 1921. Option C is wrong because Max Planck introduced the quantum of energy in 1900 to explain black body radiation, which Einstein then applied to light. Option D is wrong because Millikan's role was experimental verification of Einstein's equation and a precise value of Planck's constant. Keep observer, explainer and verifier apart.

Read the full article: Photoelectric Effect, Lasers and Superconductivity Notes

Practice Questions

View all
Q1.General ScienceMedium

Albert Einstein was awarded the Nobel Prize in Physics for 1921 mainly for

  1. A.The special theory of relativity
  2. B.The general theory of relativity
  3. C.His explanation of the photoelectric effect
  4. D.The discovery of the photon rest mass
Show answer
Correct answer: C. His explanation of the photoelectric effect

The citation for the prize refers to his services to theoretical physics and especially to his discovery of the law of the photoelectric effect, the equation in which the energy of a photon equals the work function plus the maximum kinetic energy of the emitted electron.

Q2.General ScienceMedium

The maximum kinetic energy of the photoelectrons emitted from a metal surface depends on

  1. A.The intensity of the incident light
  2. B.The frequency of the incident light
  3. C.The area of the illuminated surface
  4. D.The time for which light falls
Show answer
Correct answer: B. The frequency of the incident light

By Einstein's photoelectric equation, the photon gives energy equal to Planck's constant times the frequency, part of which is spent as the work function, so the maximum kinetic energy left with the electron rises with frequency and is independent of how bright the light is.

Q3.General ScienceEasy

The minimum energy required to eject an electron from the surface of a metal is called its

  1. A.Work function
  2. B.Stopping potential
  3. C.Ionisation energy
  4. D.Binding energy
Show answer
Correct answer: A. Work function

The work function of a metal is the least energy needed to free an electron from its surface, and it is equal to Planck's constant multiplied by the threshold frequency of that metal; caesium and potassium have low work functions, which is why they respond to visible light.

Q4.General ScienceEasy

The word LASER is an abbreviation for

  1. A.Light Absorption by Spontaneous Emission of Radiation
  2. B.Light Amplification by Stimulated Emission of Radiation
  3. C.Linear Amplification of Stored Energy by Radiation
  4. D.Light Alignment by Statistical Emission of Rays
Show answer
Correct answer: B. Light Amplification by Stimulated Emission of Radiation

The device works by stimulated emission, in which a photon passing an excited atom makes it fall to a lower level and emit a second photon identical in frequency, phase and direction, so one photon becomes two and the light is amplified.