Biologist: When bacteria of a particular species are placed in a test tube that has different areas lit with different colors of light, the bacteria move only into the areas lit with a particular shade of red  The bacteria contain chlorophyll, a chemical that allows them to produce energy more effectively from this color of light than from any other
This suggests that the bacteria detect this color of light by monitoring how much energy their chlorophyll is producing
The bacteria show no tendency to move into areas lit with blue even when those areas are lit so brightly that the bacteria's chlorophyll produces as much energy in those areas as it does in the red areas.