The refraction of light through the human eye

Like a camera, the human eye focuses light to produce sharp images. Light rays reflecting from distant objects are focused on the retina after they passed through the cornea, aqueous humor, the lens, and finally the vitreous humor. A schematic of the eye is shown in Figure 1.

As the light rays pass through these various mediums, they experience a physical phenomenon called refraction. Refraction is the change in direction of light rays as they transition between different mediums. Most of the refraction of light occurs when the difference in the refractive indices of abutting mediums is the greatest. Table 1 provides a list of refractive indices of the various components of the human eye.

Once the light rays are focused on the retina, the formed image is processed by millions of photoreceptor cells called rods and cones. These photoreceptors relay the signal to the brain via the optic nerve.

Emmetropia is the term used to describe an eye without any refractive errors. The axial length of the eye, the distance from the posterior cornea to the surface of the retina, is equal to the focal length of the eye. Two common refractive errors of the eye are myopia and hyperopia. Myopia, which is also known as nearsightedness, occurs when the eye can focus on nearby objects, but not on far away ones. The light rays reflecting from faraway objects is focused in front of the retina. Hyperopia or farsightedness occurs when the eye cannot focus on nearby objects. The light reflecting from a nearby object is focused behind the retina. Both of these refractive defects can be corrected with the use of compensating lenses.