The type of radiation that lies just outside the high-frequency end of the visible spectrum is ultraviolet (UV) radiation.

Here's why:

* Electromagnetic Spectrum: The electromagnetic spectrum encompasses all types of electromagnetic radiation, arranged by frequency (and corresponding wavelength).

* Visible Light: Our eyes can only detect a narrow band of electromagnetic radiation called visible light. This band ranges from red (lowest frequency, longest wavelength) to violet (highest frequency, shortest wavelength).

* Ultraviolet: UV radiation has higher frequencies and shorter wavelengths than violet light. It is invisible to the human eye.

Let me know if you'd like to know more about ultraviolet radiation or the electromagnetic spectrum!