TAAY – ADC – Why does water make everything look darker?
Imagine, you are taking a walk after a summer rain. The sun is tickling your face as it peaks out back from behind the clouds. The smell of petrichor is filling your lungs. You have done this post-rain walk countless times before but today you notice something new that isn’t new at all. Every time it rains, the sidewalk appears darker than when it was dry. This is normal and expected. But for some reason, today you notice. And you can’t stop noticing it. For the first time, you stop and think – what is it about water that makes everything appear darker when it’s wet?
Hello, I am Luc Boutin and we are The Alien Among You. Welcome to a brand-new type of episode, I call it Alien Dinner Conversation. In these short-form episodes, we will explore a fun, interesting everyday thing about earth you may never have stopped to think about. It will be like an alien noticing cool things about our world that we all take for granted. And just maybe, these little facts will make for good dinner conversation at your next social outing.
Today we will tackle something all of us have undoubtedly noticed but never stopped to think about – what is it about water that makes everything it touches darker?
Well, it’s actually a very simple phenomenon. But first, you must understand some basics on how light and the human eye work. We humans see things when light successfully reaches an object, bounces off that object and enters our eye. This excites cells in our retinas, called rods and cones, and this information is transferred into our optic nerve, to our brain and we miraculously perceive sight. White light contains all the different wavelengths, or in other words all the different colors, of visible light. An object that appears a certain color looks that way because it absorbs all other colors except for the one that it appears to be. For example, a red object looks red because whatever pigment is on that object absorbs all other light wavelengths except for red. The red light is then reflected back at us from that object and that object appears red to us. Objects that are white reflect the entire visible light spectrum. Objects that are black do the opposite; they absorb all the visible wavelengths of light and appear black.
The relative lightness or darkness of an object works in much the same way. It’s all about how much light comes back into our retina. An object that absorbs more overall light, will appear darker to us when compared to an object that more readily reflects light. So, a dark red object and a light red object both reflect red light, but one of them also happens to absorb more overall light and will therefore appear darker than the object that is more reflective. This more reflective object appears brighter or lighter to us. That’s the basics on how we perceive light coming into our retinas.
But back to the original question for today – why does water make things appear darker? Well from our background so far, it may already be obvious to you. It seems that water makes it that any given object that is wet absorbs more light, and thus reflects less light, than when it was dry. And when less light enters our retina from any given object, it appears darker. But what is it about water that actually causes this to happen?
Water does interesting things to light. It essentially traps light. And when it traps light, that means less light gets back onto our retinas and that object appears darker than when it was dry. It does depend on the relative roughness and color of a surface, but overall, that’s what water does. That’s why the object appears darker when wet.
You see, light travels slower in water than it does in air and that makes it behave in cool ways. What’s relevant to us in this episode is what happens to light when it enters the wet surface and tries to leave it. You want to think of the water that is making the pavement wet as a tiny little, extremely thin pond. It makes what I am about to explain easier to visualize. As light pours in from the sun, the light enters the pond. But once inside the pond, the light can get partially trapped. Light enters the “pond”, strikes the cement, reflects and tries to exit the water. When it tries to exit the water, two things happen depending on the angle in which the light hits the inner surface of the water. If the light hits the water’s inner surface straight on or close to it, most of the light will escape the water and be perceived by our eye. But if the light hits the inner surface of the water from a side angle, then the light gets reflected back down into the water. This means that some of the light that enters the thin layer of water above the cement will effectively be trapped by the water. It’s reflecting back and forth from the cement to the inner surface of the water and back again.
You could also describe this by saying that a surface being wet increases the absorption of light by that surface. And when absorption is high and less light comes back into our eye, we perceive this surface as being darker. That’s why things tend to appear darker when they are wet.
This cool reflection phenomenon is called total internal reflection, and it depends on the angle in which light hits the surface of the water. For water, the critical angle at which all the light gets reflected is 48.6 degrees. Anything smaller than that angle mostly gets through the air-water interface, but some will still gets reflected. It’s just that beyond 48.6 degrees, all the light gets reflected back. Hence the word total in total internal reflection.
You can experience this for yourself in an easy and interesting experiment. The next time you’re at lake, ocean or pool (and the water is relatively calm), go under water and look up. If you look straight up through the water column, you should notice that you can see things beyond the surface of the water. But if you look at the inner surface of the water from near parallel with it, you’ll see it suddenly acts like a mirror. Now you can’t see past the water, you can see a reflection of what is underwater in the direction that you’re looking. You can also appreciate this by looking at a fish tank from below the surface peering up at the inner surface of the water. Even easier than that, when you look across at calm lake you can see that light is mostly reflected off the surface of the water. This is the same phenomenon but viewed from the outside of the water. And even easier than THAT, you can just look it up online : just type “inner underwater surface of water looks like a mirror” or something like that. When you look at water from this perspective, you are essentially inside that tiny thin “pond” of water on the pavement.
And there you have it fellow aliens – you now know why wetting something makes it look darker. You are now armed with an interesting piece of alien dinner conversation – use it as you will!
I am Luc Boutin, we are The Alien Among You, have a great day and see you in the next episode!
Bibliography
https://www.pbsnc.org/blogs/science/why-do-things-look-darker-when-they-get-wet/
https://ece.ncsu.edu/2024/why-do-things-look-darker-when-theyre-wet/
https://science.nasa.gov/ems/03_behaviors/
https://www.britannica.com/science/total-internal-reflection
