The threshold wavelength for photoelectric emission from a metal is 5,200A0. Photo-electrons will be emitted when this metal is illuminated with monochromatic radiations from    a) 5.0 watt infrared lamp        b) 1 watt infrared lamp    c) 50 watt ultra violet lamp        d) 1 watt ultraviolet lamp

The threshold wavelength is 5200A0. For a photoelectric emission to take place the wavelength of incident light must be less than the threshold value. The wavelength of UV rays is 100−4000A0 . so Photoelectrons will be emitted when this material is illuminated with monochromatic radiation from a 50W ultraviolet lamp or 1W ultraviolet lamp.