You may feel like you can see forever when you look up at the stars at night, but the actual distance at which the naked eye can perceive light is the subject of debate. Many scientists have performed experiments using a candle flame as the “official” marker of brightness with varying results.
In 1941, a vision scientist named Selig Hecht conducted an experiment using flashes of blue-green light at the wavelength to which the human eye is most sensitive. His goal was to measure the number of photons necessary for the light to be visible. Photons emitting from a source or bouncing off objects is what makes it possible for the eye to see.
Hecht discovered that it took between 54 and 148 photons for people to detect the flash. Adjusting for actual photo absorption by the retina and dimming that occurs at a distance, he deduced that people should be able to see a candle flame as far away as 30 miles. Despite this research, the estimation of human visual distance has fluctuated widely over the years.
Looking to the Stars
In 2015, Professor Kevin Krisciunas and his colleague Dan Carona set out to settle the question of distance using the scale that measures the brightness of stars. Very bright stars have a magnitude of zero, and the number gets higher as light grows dimmer. The human eye can see stars as dim as a magnitude of six, and the scientists returned to the candle flame model to determine how far away such a light would have to be to emit a similar glow.
After performing various tests using both the naked eye and digital cameras to assess brightness, Krisciunas and Carona determined that the candle flame would become imperceptible at a distance of just 1.6 miles, a little more than half the distance to the Earth’s horizon. However, unlike Hecht’s research, these experiments only tested light that remained a steady brightness.
A Continuing Conundrum
Simply measuring how far away that humans can see light doesn’t reveal much about the distance solid objects have to be at before you can’t see it any more. It’s estimated that two miles is the maximum distance at which your eyes can make out distinct details on larger objects, but more studies will have to be conducted before an actual number is reached.
Even if these experiments are performed, the distance of human sight may continue to be in dispute. Many different conditions affect how far you can see, including dust or haze in the air, how flat and featureless the surrounding landscape is and the overall health of your eyes.
Although we may never know exactly how far the unaided human eye can see, it’s important to take good care of your eyes to preserve your vision for life. Regular visits to the eye doctor, remembering to wear sunglasses and keeping eyewear in good condition will help you to make the most of your full visual range.
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