Under black lights, bananas glow blue. Ripe bananas appear yellow to the naked eye under normal conditions due to organic pigments known as carotenoids. Chlorophyll begins to degrade as bananas ripen. This pigment is what causes bananas to glow, or fluoresce, under UV light and appear blue. While this is undoubtedly one of the most fascinating facts about bananas, there is another that will make you want to eat a banana every day.