What effect does optical density have on the speed of light?

Optical density refers to the measure of how much a material can slow down light as it passes through. When light travels through different materials, such as air, water, or glass, its speed changes depending on the optical density of the medium. In materials with higher optical density, the speed of light is reduced compared to its speed in a vacuum, which is the fastest speed light can travel.

As light enters a medium with a higher optical density, it interacts with the atoms or molecules in that medium, causing it to take longer to pass through than it would in a vacuum. This principle is foundational in optics and explains phenomena such as refraction, where light bends as it passes from one medium to another with different optical densities.

Therefore, the correct statement is that optical density decreases the speed of light as it travels through the material.