#uv light
Young blue stars circling the galactic center dominate the Andromeda Galaxy in this image taken in ultraviolet! About 2.5 million light-years away, the Andromeda Galaxy, also known as M31, really is our galactic neighbour. Spanning about 230,000 light-years, it took 11 different image fields from NASA’s Galaxy Evolution Explorer (GALEX) satellite telescope to produce this gorgeous portrait of the spiral galaxy in ultraviolet light in 2003. Its spiral arms stand out in visible light images, Andromeda’s arms are sites of intense star formation. They have been interpreted as evidence that Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago. The Andromeda galaxy and our own comparable Milky Way galaxy are the most massive members of the Local Group of galaxies and are projected to collide in several billion years – perhaps around the time that our Sun’s atmosphere will expand to engulf the Earth.
Image Credit: NASA, JPL-Caltech, GALEX
Bananas are one of the most popular fruits in the world. Love them or hate them, most of us know what they look like. Despite their global presence, few stop to think about where these fruits come from. That is a shame because bananas are fascinating plants for many reasons but now we can add blue fluorescence to that list.
Before we dive into the intriguing phenomenon of fluorescence in bananas, I think it is worth talking about the plants that produce them in a little more detail. Bananas belong to the genus Musa, which is located in its own family - Musaceae. Take a step back and look at a banana plant and it won’t take long to realize they are distant relatives of the gingers. There are at least 68 recognized species of banana in the world and many more cultivated varieties. Despite their pan-tropical distribution, the genus Musa is native only to parts of the Indo-Malesian, Asian, and Australian tropics.
Banana plants vary in height from species to species. At the smaller end of the spectrum you have species like the diminutive Musa velutina, which maxes out at about 2 meters (6 ft.) in height. On the taller side of things, there are species such as the monstrous Musa ingens, which can reach heights of 20 meters (66ft.)! Despite their arborescent appearance, bananas are not trees at all. They do not produce any wood. Instead, what looks like a tree trunk is actually the fused petioles of their leaves. Bananas are essentially giant herbs with the aforementioned M. ingens holding the world record for largest herb in the world.
When it comes time to flower, a long spike emerges from the main growing tip. This spike gradually elongates, revealing long, beautiful, tubular flowers arranged in whorls. For many banana species, bats are the main pollinators, however, a variety of insects will visit as well. In the wild, fruits appear following pollination, a trait that has been bred out of their cultivated relatives, which produce fruits without needing pollination. The fruits of a banana are actually a type a berry that dehisce like a capsule upon ripening, revealing delicious pulp chock full of hard seeds. Not all bananas turn yellow upon ripening. In fact, some are pink!
For many fruits, the act of ripening often coincides with a change in color. This is a way for the plant to signal to seed dispersers that the fruits, and the seeds inside, are ready. As many of us know, many bananas start off green and gradually ripen to a bright yellow. This process involves a gradual breakdown of the chlorophyll within the banana skin. As the chlorophyll within the skin of a banana breaks down, it leaves behind a handful of byproducts. It turns out, some of these byproducts fluoresce blue under UV light.
Amazingly, the fluorescent properties of bananas was only recently discovered. Researchers studying chlorophyll breakdown in the skins of various fruits identified some intriguing compounds in the skins of ripe Cavendish bananas. When viewed under UV light, these compounds gave off a luminescent blue hue. Further investigation revealed that as bananas ripen, their fluorescent properties grow more and more intense.
There could be a couple reasons why this happens. First, it could simply be happenstance. Perhaps these fluorescent compounds are simply a curious byproduct of chlorophyll breakdown and serve no function for the plant whatsoever. However, bananas seem to be a special case. The way in which chlorophyll in the skin of a banana breaks down is quite different than the process of chlorophyll breakdown in other plants. What’s more, the abundance of these compounds in the banana skin seems to suggest that the fluorescence does indeed have a function - seed dispersal.
Researchers now believe that the fluorescent properties of some ripe bananas serves as an additional signal to potential seed dispersers that the time is right for harvest. Many animals including birds and some mammals can see well into the UV spectrum and it is likely that the blue fluorescence of these bananas is a means of attracting such animals. Additionally, researchers also found that banana leaves fluoresce in a similar way, perhaps to sweeten the attractive display of the ripening fruits.
To date, little follow up has been done on fluorescence in bananas. It is likely that far more banana species exhibit this trait. Certainly more work is needed before we can say for sure what role, if any, these compounds play in the lives of wild bananas. Until then, this could be a fun trait to investigate in the comfort of your own home. Grab a black light and see if your bananas glow blue!
Exactly a year back on this very day, we explored how polarized sunglasses work and why Pilots prefer NOT to use them.
What about those with power prescriptions you ask ?
These days you can find a lot of people even with power prescriptions sporting glasses/lenses that turn dark when exposed to sunlight. These are known as Photochromic lenses.
Let’s do a quick demo:
This is a photochromic lens getting exposed to 395-400 nm light turning dark as a result.
How does it do that ?
To learn how this is achieved it must be understood that there are these class of reactions called ‘Reversible reactions’
A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B.
Example of reversible reaction of Bismuth Chloride
In the case of Bismuth Chloride, sunlight acts as a mediator to change it from a transparent solution to a dark colored solution and shaking in air brings it back.
In the case of Photochromic lenses, an organic dye is used that darkens under UV and lightens under its absence
Alternate reality: Photo-degradation
If you ever have had the unfortunate experience of leaving your plastic in the sun or wondered why the colors of your clothes fade away, then the technical term to express your misery is Photo-degradation.
This is the other extreme of a reversible reaction where UV light from the sun break break down the chemical bonds in the plastic and clothes (called chromophores) and eventually leads to fading of the colors in the object
The catch
As any user of one of these Photochromic lenses will tell you, there are some aspects of these lenses that still need to be improved
- The chemical reaction rate depends on the temperature of the surrounding. This means that on a sunny day if you are in a really warm climate, the lenses will adapt faster than if you live in a cold climate
- Most car windshields block UV light to some extent and therefore these lenses are not effective when you are inside the car.
- They do degrade after a while. Most of lenses do degrade having been used after a couple of years and need to be changed.
But having said that Photochromic lens are indeed a really cool piece of technology.
Have a great day!