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The phantom lighthouse illusion occurs when an observer is standing with her back to a lighthouse and perceives that the beams sweeping overhead are generated from a source located in front of her rather than behind her. If the observer is standing with her back pressed against a 30 m lighthouse (ie the separation between the lighthouse and the observer is 0 m), then no illusion is experienced:
Our Bayesian model predicts that the illusion begins to emerge when the observer is around 67 m in front of the lighthouse. Here's what the observer perceives at that distance.
If the observer is standing 300 m in front of the lighthouse, then the illusion is relatively strong:
For more about the illusion, see Kemp, C., Xu, W. & Cropper, S. J. (2026). A Bayesian account of the phantom lighthouse illusion. Open Mind. Preprint. Code and data. If you'd like to experience the illusion in the wild, one place where it is often seen is Lady Elliot Island, which lies off the coast of Queensland, Australia. On p 61 of The Visual Guide to Lady Elliot Island, Jeremy Somerville writes that the six rotating beams of the Lady Elliot lighthouse are "often mistaken for other lighthouses over the horizon. This is an optical illusion that seems to be more prevalent when there is thick cloud around." The illusion has also been documented at Belle-Île-en-Mer, which lies off the coast of Brittany, France.
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