Question

A surface has work function 3.3 $eV$ . Which of the following will cause emission?

• A. 100 $W$ incandascent lamp
• B. 40 $W$ flouroscent lamp
• C. 20 $W$ sodium lamp
• D. 20 $W$ Hg lamp

Answer 1

The correct option is D 20 $W$ Hg lamp

Since ${\lambda }_{min}=400$ is the minimum wavelength of visible region we will use it in the expression

$E=\frac{1240}{\lambda }eV$

$⟹E=\frac{1242}{400}=3.1eV.$

$\therefore$ In order for emission we require a source of uv light, since visible light can not cause emission. In the given options, only Hg lamp is capable of generating uv light.

So, the answer is option (D).