The threshold wavelength for photoelectric emission from a material is 5500   A ∘ .…

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

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

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

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