Question

Light from a discharge tube containing hydrogen atoms falls on the surface of a piece of sodium. The kinetic energy of the fastest photoelectrons emitted from sodium is 0.73 eV. The work function for sodium is 1.82 eV. The rocoil speed of emitting atom assuming it to be at rest before ionisation is (ionisation potential of hydrogen is 13.6 eV).

• A. 0.18ms^-1
• B. 1.80 ms^-1
• C. 8.10ms^-1
• D. 0.81ms^-1

Answer 1

Answer: (D)

0.81ms^-1

Answer 2

Momentum of hydrogen atom = momentum of emitted

photon

i.e. mv = $\frac{E}{c}$

or v = $\frac{E}{mc}$

= $\frac{(0.73 + 1.82)1.6 \times 10^{- 19}}{1.67 \times 10^{- 27} \times 3 \times 10^{8}}$ = 0.81 ms^-1