a light switch is 'un interrupteur' (masc.) or familiarly 'le bouton'
well most people say turn off the light but you could say switch of the light.
To say "light" as in a light color in French, you can use the word "clair."
Commutateur sur l'as.
a light skin (in the sense of "fair complexion") is 'peau claire' in French.
Lampe
leger = light
You say "turn on the light" because that is what you do. Turning on a light means to flip the electric switch from "off" (where there is no electricity and the light is dark) to "on" (where there is electricity and the light is glowing).
The English phrase, "writing with light", is "écrire avec la lumière" in French.
Ampoule - light bulb.
Feu vert
Bleu Clair .
petite lumière