Source: The Conversation (Au and NZ)
There are two lighthouses on Lady Elliot Island, at the southernmost tip of Australia’s Great Barrier Reef. The first was built in 1873, but was decommissioned in 1995 when the second took over the job of warning ships off the reef.
Visitors to the island, however, regularly report seeing signs of a third lighthouse. At night the beams of a non-existent phantom lighthouse are seen far off, shining on the horizon.
This “phantom lighthouse illusion” is so well known it even gets a mention in the island’s guide for tourists. It’s not unique to Lady Elliot Island, either: the earliest known report of this phenomenon was published in 1937, and it documented the illusion off the coast of Brittany in France.
What’s going on here? One of us travelled to Lady Elliot Island to find out, then we recreated the illusion in a virtual reality environment to make sure we understood what was happening.
Like many illusions, this one seems to happen when our brains add our expectations to evidence from our senses and arrive at the wrong result.
What the illusion looks like
To start, at night, stand at least 70 metres away from the lighthouse between the lighthouse and the sea. Look up at the lighthouse and you’ll clearly see a rotating beam of light shining from where you’d expect.
Now, turn around with your back to the lighthouse and look out to sea. You know the light is coming from behind you but you’ll be convinced what you’re looking at is the light emanating from the horizon.
It won’t matter you know this is wrong. No matter how many times you turn around to check the source, the phantom lighthouse is still there when you gaze again at the horizon.
The video below – from our virtual reality recreation – gives some sense of what the experience is like. In reality, the beam is generated by a lighthouse located 300m behind the observer. The video makes it appear as if the beam is generated by a moving source that lies in front of the observer.
Simon Cropper, CC BY
Why a lighthouse fools our brains
For our recent study, we first examined the natural illusion “in the wild” on Lady Elliot Island. Then we managed to replicate it in a fairly simple virtual reality environment – the lab results matched the real-world observations remarkably well.
Our work suggests that what people actually see is a combination of what they expect to see and the evidence provided by their senses.
At night, few distance cues are available so the evidence is sparse. This means the visual system needs to rely on expectations about the distance of different parts of the lighthouse beam.
As an analogy, imagine you are in a room that is completely dark except for a small glowing disc at eye level. The disc could be a small object that is relatively close, or a much larger object far away. Because the evidence is ambiguous, your interpretation of the scene will be mostly determined by what you expect to see.
Our study suggests the phantom lighthouse illusion works the same way. People expect points along the lighthouse beam to be much closer than they actually are.
In reality, the far end of the beam is both very distant and moving extremely fast. But another possible interpretation is that this part of the beam is not so far away and is moving relatively slowly.
If the far end is perceived as moving slower than the nearer end, the natural interpretation is that the source of the light is somewhere in front of the observer.
Prediction machines
Our account of the illusion made a novel prediction. Earlier observers reported experiencing the illusion at distances between 1–4 kilometres from a lighthouse.
By contrast, our theory predicts that a distance of around 70m should be enough for the illusion to emerge. This prediction was confirmed by our virtual study and our own experience on Lady Elliot Island.
The phantom lighthouse illusion highlights the operation of the brain as a “prediction machine” that combines expectations and current data to make inferences about the world. Normally these expectations allow us to make accurate inferences about what we see.
Astronomers are alert for the famous “crater illusion”, for example, which occurs because we normally expect that light in an image will be coming from above.
This expectation is useful in interpreting ambiguous scenes, but it can sometimes lead us astray: a photograph of a crater can easily be mistaken for one of a dome or hill instead, depending on the direction of the light.
NASA / GSFC / Arizona State University
Illusions are so fascinating to us because they are times when our brains make a mistake. By the same token, they show us how the brain actually gets things “right” most of the time.
We may get the location of a lighthouse wrong by tens of kilometres, but we do it for the same reason we correctly understand our environment almost all the rest of the time.
The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
Original source: https://analysis1.mil-osi.com/2026/10/27/the-phantom-lighthouse-new-research-explains-one-of-australias-strangest-illusions/