Here's why:

* Metals are good conductors and absorbers of electromagnetic radiation. This means they readily absorb the energy from UV rays.

* UV rays have relatively low energy compared to other forms of electromagnetic radiation like X-rays or gamma rays. While X-rays can penetrate some metals, UV rays are not energetic enough to do so.

Therefore, metal acts as a barrier to UV radiation. However, the specific amount of UV radiation that can penetrate depends on the type of metal, its thickness, and the wavelength of the UV radiation.