How is the index of refraction calculated?

The index of refraction is a measure of how much light slows down when it enters a different medium compared to its speed in a vacuum or air. It is calculated by taking the speed of light in air and dividing it by the speed of light in the material through which it is passing, such as a lens. This relationship defines how much the path of light will bend or refract when transitioning between these two media.

In the correct answer, the calculation properly utilizes the speeds of light, which are essential in determining how refractive properties differ between air and the lens material. The resulting value of the index of refraction indicates how much slower light travels in the lens material compared to air, which influences optical properties and behaviors.

The other choices involve incorrect parameters or ratios that do not represent the index of refraction. For example, using the speed of sound does not apply, as sound and light propagate differently and are unrelated in terms of refractive index calculations. Therefore, understanding the correct relationship between the speeds of light in different media is crucial for applying the concept of refraction accurately.