Mention any three features of results of experiment on photelectric effect which cannot be explained using the wave theory of light.

Question

(a) Mention any three features of results of experiment on photelectric effect which cannot be explained using the wave theory of light.

(b) In his experiment on photoelectric effect, Robert A. Millikan found the slope of the cut-off voltage versus frequency of incident light plot to be 4.12 × 10^–15 Vs. Calculate the value of Plank’s constant from it.

Answer 1

(a) Mention three features of results of the photoelectric experiment which cannot be explained using the wave theory of light.

Instantaneous emission of electrons – According to wave theory, energy is spread over the wavefront so electrons should take time to accumulate enough energy. However, photoemission is instantaneous.

Existence of threshold frequency – Wave theory predicts that any frequency of light should eventually eject electrons if the intensity is high enough, but in reality, photoemission only occurs above a specific frequency.

Dependence of kinetic energy on frequency – The kinetic energy of emitted electrons depends on light frequency, not intensity, contradicting wave theory.

(b) Robert Millikan found the slope of the cut-off voltage vs. frequency plot to be 4.12 × 10^–15 Vs. Calculate the value of Planck's constant

From Einstein's photoelectric equation:

\(ev_0 = hf - f\) 

where e is charge of an electron, V₀ is stopping potential, and h is Planck's constant.

Slope of V₀ vs. f graph \(= \frac{h}{e}\) 

Given: 

\(\frac{h}{e} = 4.12 \times 10^{-15}\)  Vs

Since e = 1.6 × 10^–19 C

h = (4.12 × 10^–15) × (1.6 × 10^–19)

h = 6.592 × 10^–34 Js