Question

The work function $\left(\varphi \right)$ of some metals is listed below. The number of metals which will show photoelectric effect when light of 300 nm wavelength falls on the metal is:___

Metal	Li	Na	K	Mg	Cu	Ag	Fe	Pt	W
$\varphi$ (eV)	2.4	2.3	2.2	3.7	4.8	4.3	4.7	6.3	4.75

Answer 1

Since the value of hc = 1240 eV nm
Energy corresponding to $\lambda$ = 300 nm is $\frac{1240eVnm}{300nm}=4.134eV$
Elements having $\varphi$ < 4.134 eV will show photoelectric effect, are Li, Na, K, and Mg $\Rightarrow$ 4
So, number of metals showing photo-electric will be four i.e., Li, Na, K, and Mg.