A human’s ability to perceive objects in an extremely wide range of light and darkness is fascinating.
But how does it all work inside of our eyes? How do our eyes adapt to being deprived of light? How does our “night vision” work?
Those questions and many more will be answered by these four amazing facts you might not know about human night vision.
1. Rods and Cones Determine What We See
There are two receptors inside the eye’s retina that collect and send signals and impulses to the brain. These impulses make up everything that we see.
These two receptors are the rod and the cone.
The rod is responsible for our light sensitivity and determines whether our pupil should expand to let light in, or contract to filter the amount of light we are receiving.
The cone is responsible for our sense of color and our sense of focus. The cone pulls double duty, since it not only has to filter into the brain what colors we perceive but also works with the brain to help focus our eyes on whatever we are looking at.
The dimmest light that the cone can focus in is roughly 50% of moonlight. While the rod can see in much dimmer conditions, the cone causes you to lose focus and the ability to perceive color in conditions dimmer than 50% of the moon’s light.
2. Your Eyes Adapt At Different Rates
Have you ever walked into a dark room and had to stand still for a couple of minutes so your eyes could “adjust” to the darkness? It turns out that this isn’t all in your head.
Rods and cones require different amounts of time in order to achieve maximum sensitivity in dark or dim conditions.
It takes the rod around 45 minutes to obtain maximum sensitivity, but the cone takes about 30 minutes to achieve max sensitivity.
3. Understand Your Blind Spot
When trying to see in the dark, you might notice that there will be a giant blind spot in the middle of your eye, making it impossible to stare straight at an object.
This is because the middle of your eye is made up primarily of cones.
If the light condition you are in is under that 50% moonlight threshold, your cones will not be able to take in any light, creating that blind spot. Meanwhile, your rods are still able to filter the dim light around your peripherals, making it possible to use peripheral vision to see in dark conditions.
4. Hold The Flash
Most of us have probably had the experience—probably while driving—of trying to use your night vision to see and being temporarily blinded by a bright light, only to return to the darkness and feel like you are unable to see.
This happens because even only a few seconds of bright light causes our rods and cones to adjust to filter out the light, virtually resetting the time it takes for them to adjust to darkness again.
If this happens to you, remember that night vision is exclusive to each eye separately—if you are exposed to light in one eye, the other can still retain its night vision if it’s not exposed.
You might have experienced these situations in the past, and instinctively, or through word of mouth, have known what was happening or what to do.
But, it is always interesting to know exactly what is happening in our bodies to make us have the reactions we are having, especially when it comes to something as far-fetched as seeing in the dark.
The next time you’re outside after sundown, thank your rods and your cones for giving you the opportunity to see everything around you, and saving you from that tree branch you’re about to trip over!