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Q.

A radiation of wavelength λ illuminates a metal and ejects photoelectrons of maximum kinetic energy of 1 eV. Another radition of wavelength λ/3, ejects photoelectrons of maximum kinetic energy of 4eV. What will be the work function of metal ?

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a

1 eV

b

2 eV

c

0.5 eV

d

3 eV

answer is C.

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Detailed Solution

hv = w + KE

KE=\frac{hc}{\lambda }-w

K{{E}_{1}}=\frac{hc}{{{\lambda }_{1}}}-w

given K{{E}_{1}}=1eV

{{\lambda }_{1}}=\lambda

1=\frac{hc}{\lambda }-w

\Rightarrow \frac{hc}{\lambda }=1+w ... (1)

\Rightarrow K{{E}_{2}}=\frac{hc}{{{\lambda }_{2}}}-w

given K{{E}_{2}}=4eV

{{\lambda }_{2}}=\frac{\lambda }{3}

4=\frac{hc}{\left( \frac{\lambda }{3} \right)}-w

4=\frac{3hc}{\lambda }-w

\frac{3hc}{\lambda }=4+w ... (2)

\frac{eq\,\,(2)}{eq\,\,(1)}=(3)

3=\frac{4+w}{1+w}

3+3w = 4+w

2w = 1

w = 1.5

work function = 0.5 eV

 

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