BLACK-AND-WHITE FRINGES AND THE COLORS OF CAUSTICS

Fine-scale interference fringes that decorate caustics produced with white light appear black and white with high contrast. This is surprising, as the elementary expectation, supported by theory and computer simulation, is that the fringes should be highly colored. The fringe separation is several arc minutes and therefore close to the resolution limit of the eye. Under magnification (even of a photograph), the colors are revealed. Therefore black-and-white fringes are an illusion, giving a dramatic naked-eye illustration of the fact that the angular resolution of the visual system is better for luminance than for color.