The essential condition for the action of a laser is
- A.Population inversion
- B.Total internal reflection
- C.Thermionic emission
- D.Nuclear fission
Correct answer
Explanation
The correct answer is A, population inversion. In ordinary matter most atoms sit in the lower energy level, so light passing through is absorbed more than it is amplified. Laser action needs the opposite situation, more atoms in the higher level than in the lower one, which is called population inversion and is achieved by pumping with the help of a metastable level where atoms can wait. Option B is wrong because total internal reflection guides light along an optical fibre but does not create or amplify it. Option C is wrong because thermionic emission is the escape of electrons from a hot filament, used in vacuum tubes. Option D is wrong because nuclear fission is the splitting of heavy nuclei and has nothing to do with producing coherent light.
Read the full article: Photoelectric Effect, Lasers and Superconductivity Notes
Practice Questions
View allThe photoelectric effect was first observed by which scientist?
- A.Albert Einstein
- B.Heinrich Hertz
- C.Max Planck
- D.Robert Millikan
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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.
Read the full explanation →Albert Einstein was awarded the Nobel Prize in Physics for 1921 mainly for
- A.The special theory of relativity
- B.The general theory of relativity
- C.His explanation of the photoelectric effect
- D.The discovery of the photon rest mass
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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.
Read the full explanation →The maximum kinetic energy of the photoelectrons emitted from a metal surface depends on
- A.The intensity of the incident light
- B.The frequency of the incident light
- C.The area of the illuminated surface
- D.The time for which light falls
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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.
Read the full explanation →The minimum energy required to eject an electron from the surface of a metal is called its
- A.Work function
- B.Stopping potential
- C.Ionisation energy
- D.Binding energy
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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.
Read the full explanation →The word LASER is an abbreviation for
- A.Light Absorption by Spontaneous Emission of Radiation
- B.Light Amplification by Stimulated Emission of Radiation
- C.Linear Amplification of Stored Energy by Radiation
- D.Light Alignment by Statistical Emission of Rays
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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.
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