The layer in the upper atmosphere that prevents most of the sun's ultraviolet radiation from reaching Earth is the stratosphere, specifically the ozone layer.

Here's how it works:

* Ozone (O3) is a molecule made up of three oxygen atoms.

* Ultraviolet (UV) radiation from the sun has enough energy to break apart oxygen molecules (O2) in the stratosphere.

* When an oxygen molecule is split, the individual oxygen atoms can then combine with other oxygen molecules to form ozone.

* Ozone molecules absorb UV radiation, preventing it from reaching the Earth's surface.

This process is crucial for life on Earth. UV radiation can cause skin cancer, cataracts, and other health problems. The ozone layer effectively filters out most of the harmful UV radiation, allowing only a small amount to reach the surface.