Einstein's Photoelectric Equation
Ultraviolet light of photon energy 5.0 eV strikes a sodium plate whose work function is 2.3 eV inside a photoelectric apparatus.
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About This Question
- Subject
- physics
- Chapter
- dual nature of matter and radiation
- Topic
- einstein's photoelectric equation
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
2.7 eV
Einstein's photoelectric equation from NCERT states that the maximum kinetic energy of an emitted electron equals the photon energy minus the work function, written Kmax=hν−ϕ. This expresses energy conservation: a single photon spends part of its energy freeing the electron and the remainder becomes the electron's kinetic energy. Substituting the given values, Kmax=5.0−2.3=2.7 eV. The value 7.3 eV is wrong because it adds the work function instead of subtracting it, violating energy conservation. The value 3.5 eV is wrong because it uses an incorrect work function value not supplied in the problem. The value 2.3 eV is wrong because it simply repeats the work function, which is the energy consumed, not the leftover kinetic energy. As stated in NCERT Class 12, Chapter 11, only the photon energy in excess of the work function appears as electron kinetic energy, and this linear relationship was Einstein's crucial insight. A magnitude check confirms that with a 5.0 eV photon and a 2.3 eV barrier, the surplus 2.7 eV sensibly represents the fastest electrons emitted.
This easy difficulty physics question is from the chapter dual nature of matter and radiation, covering the topic of einstein's photoelectric equation. It appeared in the 2025 exam.
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