How The Eyes Work In The Dark

Your eyes work in the dark in the same way they work in the light: light enters your eye through the pupil, creating an image that gets inverted by the lens onto your retina, which then travels through your optical nerve into your brain. The nuances, however, are different. Seeing in the dark involves extra work from your iris and its muscles and requires you to rely on different retinal photoreceptors. These nuances, however, are also the reason that, as you age, your night vision decreases faster than your normal vision.

The Iris & Pupil

The iris is the colored part of your eye; the iris' muscles control the size of your pupil, the dark center of your iris, allowing more or less light to enter as needed. When it is bright, your pupil is smaller, allowing in only as much light as it needs. But when it is dark, your pupil grows bigger, allowing your eye to catch as much light as it can get.

The Retina

The retina is the rear wall of your eye; it captures the inverted image that is focused through the lens and sends that image to the optical nerve. Your retina is composed of two different light-sensitive cells, known scientifically as photoreceptors. These photoreceptors, the cones and the rods, process information in different ways.

According to Georgia State University, the cones allow you to see color and respond to changes in brightness, while rods see detect only black and white but respond to shape and movement.

Rods

Rods are ineffective in bright light but will act as light detectors in low levels of illumination. They are over 1,000 times more sensitive than cones, and can often be activated by a single photon, or light particle. However, rods take more time to adjust to a lack of light; as a result, it may take 30 minutes or more to fully adjust from a bright light to a dim light.

Additionally, rods do not respond to red lights. Looking at a red exit or emergency light in a dark building will not impact your night vision, but looking at a white exit light would require your eyes to adjust again, notes Georgia State University.

Rhodopsin

When photons reach the rods in your retina, they are absorbed by protein molecules called rhodopsin. These rhodopsin molecules contain other molecules known as retinal. When the absorbed photon reaches the rhodopsin, it excites the retinal, creating a series of electrical impulses that travel to the optic nerve, which are then carried to the brain. Scientists are still studying rhodopsin and retinal to better understand their molecular actions.

Age

As you age, you tend to lose your night vision faster than your regular vision. Part of this loss of night vision spawns from a lack of rods in your eyes. Young eyes have a ratio of nine rods for every cone, but older adult eyes have only about six rods for each cone.

As you age, the muscles that control your iris become weaker; as a result, your pupils cannot expand to the size they could when you were young, reports Georgia State University.