Why Do Wet Surfaces Look Darker?

Why do wet surfaces appear darker?

There is a situation that we all face in our daily life. Sometimes we see this undesirable darkness under the armpits of our t-shirts on a hot day, and sometimes our clothes get darker because of the splashing water while washing our hands. At times, we may even have to prove that the splashing water on our trousers is not due to other causes. Of course, there is a scientific explanation for this unpleasant situation. So let’s get straight to the point:

How is Image Formed?

The interesting thing is that even transparent water will darken a fabric when it comes into contact with it. If it was a dark liquid, this situation might be a little more plausible, but we can observe this phenomenon even in transparent liquids. No matter how good the fabric is (except for waterproof fabrics), we see this darkening more or less. In other words, this situation has no direct connection with the quality of the fabric. In fact, all the blame lies in optical reflections. To better understand this situation, let’s look at how the image is formed.

Image Generation

In order for our brain to perceive an object, light must fall on that object and this light must be reflected in our eyes. Brighter ones reflect light more, while duller/dark ones reflect less light. In other words, it absorbs some of the light.

Why Does Water Darken Fabric?

When the fabric is wet, water molecules enter the fabric and form an additional layer. The rays hitting the water layer inside the fabric have to pass through this layer before they reach our eyes. However, it is not easy to pass through the water layer that fills the gaps in the fabric. Some of it is reflected back before it reaches our eyes, and therefore the surface appears darker.

In short, less light reaches our eyes because the layer of water added to the fabric absorbs some of the reflected rays. Less reflected light causes the surface to appear darker.

sweaty man

Why Doesn’t the Same Case Happen for Spilled Water on the Table?

In fact, we observe this darkening, albeit to a lesser extent, for water poured onto a dark table. But to introduce two new concepts, let’s assume that there is hardly any darkening and ignore this situation.

Although the water spilled on wooden or plastic tables remains on the surface, it does not appear dark. The reason for this is that the process of absorbing water from these surfaces is quite slow. Water is absorbed in a short time on surfaces such as fabric. For this reason, the reflection of light becomes difficult. However, even if the reflected light is slightly refracted on surfaces, it is not affected by absorption. For this reason, it does not appear darker.

To better understand and reinforce this situation, let’s define two terms that are often confused with each other: Absorption and Adsorption.

We can think of absorption as the immersion of the liquid into the surface it comes into contact with and almost becoming one with it. As an example, we can imagine the sponge. The sponge absorbs the liquid and almost swallows the liquid.

Adsorption is when a liquid is only on the outside of the surface it comes into contact with. In other words, there is no absorption and integration of the liquid this time. The liquid remains only on the surface.

Now let’s think again about the fabric and the table. In the fabric example, just like the sponge, water molecules dive into the fabric and integrate into it. Therefore, the rays hitting the wet fabric lose their power between the fabric filled with water molecules, and not enough rays can reach our eyes. This phenomenon is called absorption and this is why we see the fabric as dark.

Wet Table

However, in the sample of water spilled on the table, the water molecules remain on the table to a great extent.  There is no integration with the table. Therefore, the light first encounters the liquid and then reaches our eyes with less refraction. For this reason, the water spilled on the table appears less dark to us than the fabric.

Again, of course, some refraction occurs due to the water on the surface. Therefore, there is some darkening. However, this darkening is much less than that of the fabric.

Note: The good news is liquids can evaporate at any temperature. Water spilled on you for some reason will evaporate after a short time, causing the dark color to disappear and return to its original state. Of course, there are also liquids that are more stubborn than water. Therefore, the darkness caused by the liquids spilled on you will vary depending on the content of the liquid.

References and Further Readings

Matthews, R. (n.d.). Why do clothes get darker when wet? BBC Science Focus Magazine.

What is Adsorption? The International Adsorption Society. (2018, November 8).

Woodward, A. (2018, May 18). Why Do Things Get Darker When Wet? LiveScience.

You can access the sources of the images used by clicking on the images.

The proofreading has been done by Asu Pelin Akköse and Mete Esencan.

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Mete Esencan

Hello everyone! I'm Mete Esencan. I am a graduate student in the Department of Chemistry at METU. I was planning to establish a platform by combining the research knowledge I gained during my basic science education and the management experience I gained in the METU Chemistry Society, which I was in charge of for three years. For this purpose, in February of 2021, I took the first step and established the OkButWhy, a platform where we can write articles as if to chat about science, art and philosophy. I wish everyone a pleasant reading!

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