The phantom lighthouse: New research explains one of Australia's strangest illusions

Lady Elliot Island, located at the southern end of Australia's Great Barrier Reef, is home to two actual functioning lighthouses. However, night visitors often claim to see what appears to be a third lighthouse, a phenomenon known as the phantom lighthouse illusion. This illusion, well-documented since 1937, even appears in a tourist guide for the island. The phenomenon extends beyond Lady Elliot Island, as similar reports have been recorded off the coast of Brittany, France.

To understand this illusion, researchers traveled to Lady Elliot Island and recreated the experience in a virtual reality environment. They discovered that the illusion occurs when observers stand at least 70 meters away from the lighthouse, with their back to the structure, and gaze out at the sea. Despite knowing the light should be coming from behind them, observers are convinced they are seeing the light emanate from the horizon.

The illusion works because the brain combines expectations with sensory evidence to form interpretations of the world. In this case, with limited distance cues at night, the visual system relies heavily on expectations about how far apart different parts of the lighthouse beam should be. Since the distant end of the beam appears both very far away and moving rapidly, observers naturally assume the light source is located in front of them.