How can we see a light across the universe if brightness diminishes with distance?

Image: James Wheeler via flickr
Image: James Wheeler via flickr

The North Star (Polaris) is a colossal 323 light years from Earth, and yet it is clearly visible in the night sky. The reason it is possible to see a star nearly 2,000 trillion miles away is that your eyes are far more sensitive than you may realise.

Light sources, whether they are something as small as a light bulb or as large as a sun, obey what is known as the ‘inverse square law’. The further we travel from a source of light, the dimmer it gets: if we double our distance from a light, it will appear one quarter as bright. And someone standing on Jupiter, which is 5.2 times further from the sun than Earth, will see the sun as 27 times dimmer!

The wonder of how we see light is due to the light sensitive part of our eye called the retina. This inner lining of the eyeball is covered in millions of tiny light detectors, which come in two forms: ‘cones’ that detect coloured light, and ‘rods’ that are responsible for our night vision. Rods are much more sensitive than cones, which explains why we cannot see colours in very dark rooms, but may still be able to make out shapes.

Back in 1979, Baylor and Lamb showed that even a single particle of light (known as a ‘photon’) was enough to trigger a response from a rod cell in a toad’s eye. For the retina to send this information to the brain and allow us to see the light, however, several rods have to be ‘activated’. A single photon is not enough to do this, but the threshold is still surprisingly low. Copenhagen and Reuter found that fewer than 100 photons entering the eye were enough for rod-mediated vision.

The upshot of this science is that we can see brightly lit objects from unimaginably large distances, like the North Star or the Andromeda Galaxy (which is a whopping 2.5 million light years away and is the most distant object visible to the naked eye). So sensitive are your eyes that on a dark night, you can even see the flicker of a humble candle from up to 30 miles away!

The human eye is truly a thing of wonder (it can even detect polarised light!). So stop hiding your light (sensors) under bushel and embrace the amazing abilities of your eyes.


By Michael McKenna


Education Gateway. (July 2004). How Far? How Faint? NASA Education. (Accessed 13 March 2015).

Baylor, D. A., (1979). Responses of retinal rods to single photons. The Journal of Physiology, 288: 613-634.

Ricky Sethi. (October 2000). What is the smallest number of photons that humans can see? MadSci Network. (Accessed 13 March 2015).

Natalie Wolchover. (May 2012). How Far Can the Human Eye See? Live Science. (Accessed 13 March 2015).

Article by Michael Mckenna

March 13, 2015

Mike is currently doing a PhD in biochemistry at the University of Manchester. When not talking about proteins, he watches an obscene amount of films and enjoys the odd pub quiz.

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