The threshold wavelength for photoelectric emission from a material...

Given here, threshold wavelength ${\lambda}_{th} = 5500Å = 550nm$

For photoelectric emission, the wavelength of the light should be less than $550nm$.

Wavelength of infrared radiation extends from $700nm - 1mm$ and that of ultraviolet radiation is $100 - 400nm$.

Since, UV light has less wavelength than $550nm$, so only UV would be able to emit photoelectrons.