Schuergers et al. illuminated Synechocystis cells on an agar surface with a light source from the direction marked with a red arrow. Each cell, acting as a lens, focused the light on the rear edge of the cell (white arrows), and moved toward the light. A strong laser light from above (red circle in center), caused cells entering the small laser-illuminated area on the agar to reverse direction. (See figure 4 in Schuergers et al., 2016 for full details).