The mirrored triangles illusion: on the perceived distance between triangles in mirror image arrangement

Abstract

The mirrored triangles illusion is described and investigated. Two black isosceles triangles are arranged symmetrically with respect to a vertical plane such that the apices face each other. While keeping their distance as well as their area constant, the shape of the triangles is varied. It is observed that the apparent distance of the tips varies as a function of the triangles´ shape, exhibiting a maximum when they are nearly rectangular. This coincides with the situation where the diameter of the outer circle encompassing both triangles approaches a minimum. An explanation is proposed where the variation of the apparent distance is ascribed to a size constancy effect. A mathematical function is derived and fitted to the experimental values. From this, a size constancy parameter of k= 0.477(33) is obtained.