What bumblebee and UV detector have in common (a zoo story)

It was a very, almost too hot day in the Dresden Zoo. Since most of the animals were just as lazy as the visitors, children read more signs that day than usual. Especially signs that stood in the shade. A very old sign about bumblebees that I had never noticed before stood on a lawn next to a bench that was placed under a tree. Shortly after I arrived at a good position under this tree, I was confronted with a scientific question: "Why can bumblebees not see red color?"

I quickly tried to answer this question without scientific content: "Most animals see the world differently than we do, some animals only might see black and white, some animals might not see all colors – or colors in a different way as we see them – and some animals see something like colors we cannot see. Look, a bumblebee cannot see red but ultraviolet."

For a brief moment, one could hear a giant turtle wandering through the lawn in the background. "I also know violet," responded my nine-year-old child. "If you really, really, really want to know more, we should sit down," I said, "here, have a cheese sandwich and something to drink. It might take some time to tell this scientific story".

This was the beginning of the theory of colors

"Look, the drawing on the sign shows different colors from red to orange, yellow, and green to blue. Above red 800 nanometers and above blue 400 nanometers is written. These nanometers stand for so-called wavelengths, in this case really lengths of light waves measured in a very tiny meter scale, much much smaller than the millimeter which is already smaller than the centimeter you know." "Yes, we already discussed millimeter in school." "That is good but let us not talk much about nanometers themselves for now. Much more important is that such wavelengths refer to different sorts of colored light beams. Do you remember what happens when a rainbow appears in the sky?" "Yes, it occurs when the sun shines and it rains at the same time." "Yes, that's the requirement. The sunlight is split by the rain into these different sorts of colored light and the rainbow appears."

"Split like wood with an ax?" "Even nastier, light is split into its constituent parts. If you take your cheese sandwich, this would not simply be cut in half. It would be all cheese being separated from all bread, from all butter and from the tomato slice - finally all different parts from each other. Afterwards, the individual components of the sandwich are on different heaps, discretely separated from each other, all heaps lying next to each other in a row.”

“Okay, this makes sense. Like the colors on this pictures, one color next to the other, in an order like the rainbow.” “Exactly! Let us talk again about the sunlight which is often referred to be yellow - but that is not correct. The sun appears to be yellow but the sunlight is not. Sunlight is called to be white in science, but that's not really true anyway – at least, it is not easy to understand. The light of the sun around us is actually seen as transparent rather than white. This transparent light is nothing but so-called waves of electromagnetic radiation. One also says light beams. Sounds pretty complicated, right?"

"Does that have anything to do with the radiation in our microwave?" "Well, somehow yes, but that is another interesting story. Let us stick to the rainbow. We are already taking a bigger explanation loop and I want to answer your initial question. Let us resume: Sunlight is split by rain into its components, so we see the colored lights in a rainbow. Do you remember the rainbow colors?" "Yes, these are all colors from red to orange, yellow, green to blue. Oh, those are the same colors as in the picture!" "Exactly. Well recognized."

Discussion about color recognition

"But you did not yet answer why the bumblebee does not see any red color." "Well, the bumblebee cannot see rays of light that we humans see as red, for example in the rainbow. The light color red refers to light beams with wavelengths from 650 to 800 nanometers as shown on the drawing. What do we need to see something?" "Stupid question, our eyes, of course." “Yes, exactly, and bumblebee eyes do not only look different to ours, they also work or see different to ours. In every eye, there are many, many visual cells, which – when talking about our human eyes – roughly refer to only three colors: Blue, green and red.

"That is not true. I know and see more colors. Look, this yellow flower, for example!" "Our human brain detects through our eyes how much light of each of these three colors, so blue, green and red, is registered in our eye. The brain then calculates which color we actually "see". Surprisingly, when we see something yellow, like that flower, the blue light beams are simply missing in the light that comes into our eye, so or brain says:" "Yellow!" "Exactly! If we see something white, then all the blue, green and red light beams are equally detected by our eye." "Why do we see so many more colors and not just white, blue, green, yellow, and red?"

"Good question. This becomes now a bit complicated to explain in words and without drawings. I just wanted to point out that humans have a sensor for red light in their eyes." (One could see my child obviously start preparing to ask for more information.) "Okay, you will ask anyway for this. Shall I really continue with this additional topic?" "Yes, I want to know it as well as these two children next to us. My eyes d-e-t-e-c-t-e-d that they are listening." (One could hear kids laughing for some seconds.)

"People can distinguish a lot of colors and we see every color from very light to very dark until the color turns black. This has to do with how much light arrives into our eye. This may have something to do with how much light is actually in the room. In the evening, it gets darker and darker, until everything is gray and later black which was previously colored. You probably know the color box rules: When you mix blue with yellow, green is created. The more yellow, the brighter the green?" "Yes, every baby knows that."

"Seeing colors is different. As I said, when we see something yellow, blue light beams are missing in the light that comes into our eye. Yellow light and blue light of the same intensity being mixed do not result in green light, it turns into something like colorless light, so-called "white" light, as our eyes now detect also blue next to green and red light beams. "Yes, this makes now sense although this is freaky as well." "Remember, with the scientific expression "white", colorless and transparent is meant for light – or really white for an object." (Five children and two adults standing and sitting next to me had big question makes in their eyes.)

"Our eye recognizes whether light is emitted from an object and, if so, also the proportioning of blue, green and red light beams in this emitted light. You can compare that proportion profile to a fingerprint, an individual color code profile which our brain converts into the color we "see". Any color has it′s own, individual fingerprint. Each small change in the blue-green-red proportioning profile results in a different fingerprint which, in turn, results in another color. This is why there are so many expressions for, e.g., the color red: Scarlet, vermilion, crimson, purple, and maroon or permanent, flame, center, blood ... even brownish red."

"Got it. But how does a tomato send out red light?" "Good question. Of course, a tomato is not a lamp. But you see a tomato only when light is "on"; this can be a lamp, the sun or any other light source. And you only see tomatoes in bright red when there is enough light.” “I remember that tomatoes looked different when I was in a supermarket – and when we had them in our kitchen, they did not look that red anymore.”

“Well recognized. The tomato reflects light of red wavelengths from a light source like a mirror reflects light – or better explained, like reflectors on your bike only shine when they are illuminated by a light source. Light beams of green and blue wavelengths are picked up by the tomato, which is also called light absorbing. Or in other words, the tomato does not reflect much of green and blue light. So we see a tomato as colored red because it reflects mainly red light. In supermarkets, they often use special light and a tomato looks deliciously red which means really tasty for us humans, so we want to buy them. It is just a trick."

"But we did not yet answered the question why the bumblebee does not see any red." "Contrary to us humans, bumblebee eyes do not have a sensor for red light but, instead, a sensor for ultraviolet light. Look at the picture: "Left" of red is infrared, "right" of blue is ultraviolet light. Hence, the bumblebee sees light beyond blue, the so-called UV light. We humans cannot see that light. But we humans have devices for that, the so-called UV detectors. These are basically bumblebee eyes recreated with technology. Your dad is using such detectors and trains other people how to use them and what to consider when using them" "That is cool... Dad, I also want to become a bumblebee-eye-device trainer."