CBSE Class 12 · 2012 · Modern PhysicsShort Answer

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Write Einstein's photoelectric equation. State clearly how this equation is obtained using the photon picture of electromagnetic radiation. Write the three salient features observed in the photoelectric effect which can be explained using this equation.

Answer: $K_{ma x} = h ν - ϕ_{0}$ ; explains threshold frequency, $K_{ma x} \propto ν$ , instantaneous emission.

Solution

Einstein's photoelectric equation:

K_{ma x} = \frac{1}{2} m v_{ma x}^{2} = h ν - ϕ_{0} .

In the photon picture light of frequency

ν

arrives as quanta each of energy

h ν

. A single photon is absorbed by a single electron; part of the energy, equal to the work function

ϕ_{0}

, frees the electron from the metal, and the rest appears as its maximum kinetic energy

K_{ma x}

.
Three features explained:
(i) a threshold frequency

ν_{0} = ϕ_{0} / h

exists; (ii)

K_{ma x}

depends linearly on frequency and is independent of intensity; (iii) emission is practically instantaneous.

K_{ma x} = h ν - ϕ_{0}

CBSE Class 12 Board · 2012 · Subjective

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CBSE Class 12 · 2012 · Modern PhysicsShort Answer

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