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 little tendency to move into areas containing light in colors other than the particular shade of red, even if their chlorophyll can produce some energy from light in those colors.