Threshold wavelength of the metal, λ0=5200Å =5200×10−10m Frequency of the radiation, v0=
c
λ
=
3×108
5200×10−10
=0.57×1015Hz =5.7×1014Hz The frequency of infrared radiation is lower than the threshold frequency of the metal. Hence, the surface of the metal is illuminated with monochromatic radiation which is greater than 5.7×1014Hz. The 50W UV-lamp is suitable for this purpose.