Images in Denser Media
If the light leaving a lens travels in a medium in which the speed of light is less than its speed in air, such as a watery tissue like the aqueous or vitreous humor of the eye, we must modify the image vergence term in the vergence equation. Here
V = η/v
where η is the refractive index of the denser medium: the ratio of the speed of light in a vacuum (or air—they are nearly the same) to the speed of light in the “denser” medium. The refractive index is always a number greater than 1.00, as light travels more slowly in dense media than in air. For the watery tissues in the eye, the value η = 1.33 is a useful approximation. (This casual use of the term “denser” has nothing to do with the specific gravity of the medium!)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.