Optician License Practice Exam

The speed of visible light travels through different media at varying rates due to the refractive index of the material. In a vacuum, the speed of light is approximately 300,000 kilometers per second, which translates to about 186,000 miles per second. However, when light moves through water, it slows down due to the denser molecular structure of the water compared to a vacuum.

In water, the speed of light is approximately 140,000 miles per second. This reduction in speed is attributed to the refractive index of water, which is about 1.33. The precise slowing down of light in water is an essential concept in optics, as it impacts how we perceive images through lenses and other optical devices. Hence, the correct choice reflects the proper understanding of light behavior in mediums other than a vacuum.