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Optics and photonics

Optics and photonics

How far away can you see light from a candle?

07 Aug 2015

 

By Andrew Silver

Can the unaided eye see the light from a single candle from 10 miles away? According to some claims on the Internet, the answer is yes – but now two scientists in the US have borrowed techniques from astronomy to show that a pair of binoculars would probably be needed.

The story behind this work began high in the Andes one moonless night when a candle was lit on the Cerro Tololo Inter-American Observatory telescope catwalk. Somebody walked 400–600 m away and said the flame was as bright as the brightest stars in the sky. Nobody wrote down any numbers.

Kevin Krisciunas, an astronomer at Texas A&M University, heard about the experiment and tried it in the suburbs. There was too much light pollution, so he thought about using the Texas A&M Observatory.

Don Carona, the director, instead offered a CCD camera to help measure the brightness of a flame against a highly visible magnitude zero star, Vega. The duo came up with a rule to describe the brightness relationship, adapting it to compare a candle flame to the faintest star an average unaided human eye can see: a magnitude-six star.

Carona and Krisciunas then turned their attention back to candle spotting. Various sources on the Internet suggest that a candle is visible to the unaided eye at distances varying from 3.6 to 30 miles. Such claims might come from 1940s work out of Columbia University that looked at how the eye responds to flashes of light in a dark room. This is a different question, says Krisciunas, who adds that watching a candle flicker isn’t the same as observing flashes of light.

By comparing the candle flame to a magnitude-six star, the researchers discovered that you would need 7 × 50 binoculars to see a candle 10 miles away. Furthermore, the farthest from which an average unaided human could see a candle is about 1.6 miles.

“It’s a pretty good estimate,” says UC Davis astronomer Maruša Bradač, who was not involved in the work.

However, this distance assumes sky observation and the atmosphere thins out as altitude increases allowing more light to pass through it. A ground based observation would have to contend with a thicker atmosphere so 1.6 miles is probably an overestimate.

A distance of 1.6 miles could be tested, although it would be difficult. So if any physicsworld.com readers can find somewhere with a clear horizontal line of sight of 1.6 miles, perhaps they could light a candle, back up and watch. And of course, share their results here by leaving a comment.

Carona and Krisciunas report their findings in a preprint on the arXiv server: “At what distance can the human eye detect a candle flame?”.

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