Question

The work function of a metal is $1.6 \times 10^{-19} J$. When the metal surface is illuminated by the light of wavelength $6400 \mathring{A}$ then the maximum kinetic energy of emitted photoelectrons will be (Planck's constant $h=6.4 \times 10^{-34} Js$

• A. $15 \times 10^{-19} J$
• B. $2.8 \times 10^{-19} J$
• C. $1.5 \times 10^{-19} J$
• D. $4.5 \times 10^{-19} J$

Answer 1

Answer: (C)

$ W=1.6 \times 10^{-19} J =1 eV$
$\frac{h c}{\lambda}=W+K_{ max }$
$\lambda=6400 \mathring{A}=640 nm ; h c=1242 eV - nm$
$\Rightarrow \frac{1242 eV - nm }{640 nm }=1 eV +K_{\max }$
$\Rightarrow K_{\max }=(1.94 eV )-(1 eV )$
$=0.94 eV \times\left(\frac{1.6 \times 10^{-19} J }{1 eV }\right)=1.5 \times 10^{-19} J .$