Certified Ophthalmic Technician (COT) Practice Test 2026 - Free COT Practice Questions and Study Guide

The index of refraction measures how much light is slowed down as it passes through a medium compared to its speed in a vacuum. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. The correct formula for the index of refraction is derived from this definition, which can also be expressed as follows: the speed of light in air (a medium close to a vacuum) divided by the speed of light in a specific substance.

This concept is fundamental in optics, as it helps in understanding how light behaves when it enters different materials. The index of refraction is crucial for various applications including lens design, understanding optical systems, and numerous other fields within ophthalmology and physics.

The other options involve either the speed of sound, which is not relevant to the calculation of the index of refraction in optics, or they misrepresent the relationship between the speeds of light in different media, which do not correctly reflect how the index of refraction is calculated. This solid understanding of light's speed relative to different media forms the foundation for more advanced topics in optics and vision science.