Atoms and molecules in their gas state cannot REFLECT light. They are not like mirrors. They are too spread out to do that. They can ABSORB light – that’s all. Ozone blocks ultraviolet light by absorbing ultraviolet light.

You will learn in Module 4 that atoms can only absorb fixed wavelengths of light, because the way they absorb energy is for their electrons to jump into higher energy orbits. Since those orbits have distinct energies, if light of many energies is shined on an atom, only those energies corresponding to the energy differences between one orbit and another will be absorbed. The rest of the energies will pass right through the atom.

In molecules, the fact that the electrons are shared tends to blur this characteristic a bit. A molecule can only absorb a certain RANGE of energies. If light within that range encounters a molecule, it will be absorbed. If light outside that range encounters a molecule, it will pass right through the molecule. Oxygen absorbs light that is higher in energy than ultraviolet light. Thus, the ultraviolet light passes right through it.