The effect of decreasing incubation time on optical density is that optical density decreases. Incubation time and optical density have a proportional relationship.

optical effect

What a charming question! But since the rainbow has no physical presence, it contains nothing.

It is just an optical effect. Another optical effect could be that of a shadow, which really has no presence or activity.

Optical Augmentation (OA) is the methods behind detecting optical devices, such as rifle scopes, cameras, binoculars, etc. An active light source is used to illuminate optical devices, which in return give off a "cats-eye" effect, giving away their position.

Luster is an optical phenomenon, the effect of interaction between light and a crystal.