Question

Five elements $A,B,C,DandE$ have work functions $1.2eV,2.4eV,3.6eV,4.8eVand6.8eV$ respectively. If light of wavelength $4000A^{\circ }$ is allowed to fall on these elements, then photoelectrons are emitted by

• A. $A,BandC$
• B. $A,B,C,DandE$
• C. $AandB$
• D. $OnlyE$

Answer 1

The correct option is C $AandB$

The threshold wavelength, ${\lambda }_0=\frac{hc}{W}=\frac{12400}{W\left(ev\right)}Angstrom$

so for $A$ ${\lambda }_0=\frac{hc}{W}=\frac{12400}{1.2}Angstrom=10333A^o$

and for $B$ ${\lambda }_0=\frac{hc}{W}=\frac{12400}{2.4}Angstrom=5166.66A^o$

for $C$ ${\lambda }_0=\frac{hc}{W}=\frac{12400}{3.6}Angstrom=3444.44A^o$

for $D$ ${\lambda }_0=\frac{hc}{W}=\frac{12400}{4.8}Angstrom=2583.33A^o$

for $E$ ${\lambda }_0=\frac{hc}{W}=\frac{12400}{6.8}Angstrom=1823.52A^o$

We see that $4000A$ is only less than ${\lambda }_0$ for $A$ and $B$