Fluorescence

Some minerals glow or fluoresce in the dark. A few of them, particularly fluorite, after which this property was named, always fluoresce. Other minerals will only fluoresce if they contain impurities such as manganese in calcite. Some minerals, when bombarded with invisible ultra-violet light will absorb it. This knocks their moveable electrons out of position. The excited electrons on returning to position give out energy in the form of light of a longer wavelength, which is visible to the eye as a glow.

A lamp in the picture is producing ultra-violet light which the eye cannot detect. A filter is cutting out as much of the unwanted, visible rays as possible, so that the fluorescent glow can be seen.

The darker this world becomes, the greater the light which individual believers takes with them. Christ Himself is the light of the world, but we are His little moons, reflecting His luminance in those dark places to which we go.