Under what circumstances will light continue to travel indefinitely?

Light will continue to travel indefinitely unless it encounters a substance that causes it to be reflected, refracted, or absorbed. In a vacuum or a transparent medium where there are no obstructions, light can propagate without limits.

When light is absorbed, its energy is converted into another form, and it no longer continues as light. Reflection changes the direction of light but does not allow it to keep traveling in a straight line indefinitely. Refraction bends the path of light as it passes through different media, altering its course, but it doesn't allow the original light to proceed indefinitely in a straight line.

Therefore, the circumstances that allow for the infinite travel of light are precisely when it is not interacting with any material that would lead to these changes. This understanding emphasizes the importance of medium in the behavior of light and clarifies the conditions under which light can propagate freely.