Rainbow in Maoz Haim, Israel

Rainbow is created when the sun light hits the rain drops. The rainbow is an excellent demonstration in everyday life of the fact that the visible light is composed of several wavelengths which correspond to the typical colors of the rainbow.  In the question, you learned that a special dye can be added to a body of water as a means of chemical reduction of Cyanobacteria bloom.  It was explained to you that this dye blocks the light from passing through the water. But, actually its exact mechanism is just blocking the red and blue wavelengths (colors) of the light while allowing the other wavelengths of the light passing through the water. The blue and red wavelengths are required for photosynthesis to occur. Hence, their blockage by the dye prevents the Cyanobacteria to perform photosynthesis. As they can no longer manufacture their food, the Cyanobacteria stop proliferating and their bloom is reduced.