Not knowing where a person is looking makes us uneasy. For this reason, it can be awkward to converse with somebody who is wearing dark sunglasses. And it is why someone might wear dark sunglasses to look “mysterious.”
A recently identified visual illusion takes advantage of the unsettling effect of uncertainty in gaze direction. The “ghostly gaze” illusion, created by Rob Jenkins of the University of Glasgow in Scotland, was awarded second prize in the 2008 Best Illusion of the Year Contest, held in Naples, Fla. In this illusion (left and center), twin sisters appear to look at each other when seen from afar. But as you approach them, you realize that the sisters are looking directly at you!
The illusion is a hybrid image that combines two pictures of the same woman. The overlapping photos differ in two important ways: their spatial detail (fine or coarse) and their direction of gaze (sideways or straight ahead). The images that look toward each other contain only coarse features, while the ones that look straight ahead are made up of sharp details. When you approach the pictures, you are able to see all the fine detail, and so the sisters seem to look straight ahead. But when you move away, the gross detail dominates, and the sisters appear to look into each other's eyes. See an interactive demo at http://illusioncontest.neuralcorrelate.com/2008/ghostly-gaze.
In another example of a hybrid image (right), a ghostly face appears to look to the left when you hold the page at normal reading distance. Step back a few meters, however, and she will look to the right.