Which one of the following statements on photoelectric effect is NOT correct ?
- A.Albert Einstein received the Nobel Prize in Physics for explaining photoelectric effect
- B.For each metal, there is a threshold frequency ν₀ below which this effect is not observed
- C.At a frequency ν > ν₀, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light
- D.The number of electrons ejected is proportional to the intensity of incident light
Correct answer
Explanation
The correct answer is C. Above the threshold, the kinetic energy of the ejected electrons rises as the frequency of the light rises, so the statement that it does not change is wrong.
Einstein's photoelectric equation is K.E.max = hν − φ, where hν is the energy of one photon and φ is the work function of the metal. Each photon gives its whole energy to one electron, so a higher frequency means a more energetic photon and a faster electron. Light below the threshold frequency ν₀ = φ/h cannot free any electron, however bright it is. Brightness, or intensity, only decides how many photons arrive, and therefore how many electrons come out.
Option A is correct as a statement: Einstein received the 1921 Nobel Prize in Physics for explaining the photoelectric effect, not for relativity. Option B is correct, since every metal has its own threshold frequency. Option D is correct, because more intense light ejects more electrons without making them faster.
Exam tip: frequency decides the energy of electrons; intensity decides their number.
Practice Questions
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