Threshold wavelength for a metal is 5200 Å. The photoelectron will be ejected, if it is irradiated by light from:

50 W infrared lamp 1 W infrared lamp 50 W ultraviolet lamp 0.5 W infrared lamp

50 W ultraviolet lamp

$Wavelength ∝ \frac{1}{Energy}$. Hence, wavelength should be smaller than threshold wavelength.