The Spanish word 'luz' meaning 'light' comes directly from the Latin word 'lux', which also meant 'light'. This is a straightforward evolution where the Latin '-x' ending changed to '-z' in Spanish, a common sound change pattern between these languages.

Some related Spanish words include 'lucir' (to shine), 'lucido' (bright, brilliant), and 'luciérnaga' (firefly). All these words share the same Latin root 'lux' and relate to concepts of light and brightness.

Many English words are related to the same Latin root 'lux', including 'lucid' (clear, bright), 'illuminate' (to light up), 'translucent' (allowing light to pass through), and 'luminous' (giving off light). The scientific unit of illumination, the 'lux', also comes directly from this Latin word.