In which medium is the index of refraction lowest?

The index of refraction is a measure of how much light slows down as it passes through a medium compared to its speed in a vacuum. The lower the index of refraction, the faster light can travel through that medium.

In this context, air is the medium where the index of refraction is lowest. This is because the composition of air allows light to travel through it with minimal slowing compared to other materials like water, glass, and plastic.

For instance, the index of refraction for air is approximately 1.0003, which is very close to 1, indicating that light travels almost at its maximum speed in this medium. In contrast, water has an index of refraction around 1.33, glass ranges between 1.5 to 1.9, and plastics can vary widely but are generally higher than that of air, typically around 1.5 to 1.7.

Hence, air provides a much less dense medium for light propagation compared to the other options, resulting in the lowest index of refraction among the choices presented.