Question

The work function of three photosensitive materials used to build photoelectric devices are given as : Sodium ( $2.75eV$ ), copper $(4.65eV)$ and gold $(5.1eV)$. Which of the following statements is correct. (The frequency of visible light lies in the range $4\times 10^{14}Hz$ to $8\times 10^{14}Hz$ )

• A. Devices built by copper and gold can operate with visible light
• B. Devices built using sodium can operate with ultraviolet light
• C. All the devices can operate with infrared light
• D. All the devices can operate with visible light
• E. No device can operate with visible light

Answer 1

Answer: (B)

Given, work function are as,
$W_{Na}=2.75eV$
$W_{Cu}=4.65eV$
and $W_{Au}=5.1eV$
The frequency range of visible light $4\times 10^{14}Hz$ to $8\times 10^{14}Hz$, so the maximum energy of photon and minimum energy of photon in visible light.
$E_{\max }=\frac{6.63\times 10^{-34}\times 8\times 10^{14}}{1.6\times 10^{-19}}(\because E=hv)$
$=3.31eV$
Similarly, $E_{\min }=\frac{6.63\times 10^{-34}\times 4\times 10^{14}}{1.6\times 10^{-19}}$
$=1.65eV$
Now, comparing the work function of above mentioned metal with $E_{\max }$. We get only sodium device can give photoelectron.
Further sodium device can also work with photons of higher energy then $2.75eV$ like $x$ -rays, UV rays etc.