Fig. 6

The difference between refractive indices of a transparent object and its mounting medium is crucial for the object’s visibility: a Left above: A gel bead is surrounded by air; b middle above: the same bead is half sunk in water—only its upper part, which is surrounded by air is visible; c right above: the same bead is completely sunk in the water and is not visible, because refractive indices of the bead and water are too similar. d Bottom left: Two coloured beads and one transparent bead are surrounded by air. e Bottom right: The three beads are completely sunk in water—only the two coloured ones are visible with blurred edges. The edges are blurred because there is only colour contrast and not phase contrast,