We have all seen how our garments can turn into a darker hue once they get soaked by water and other liquids, such as when accidentally spilling water on ourselves or after getting some exercise. In certain situations, such as during a date or a meeting, we would often wish that moisture did not have such an embarrassing effect on our clothes, but alas. If you have ever wondered about the underlying cause behind this everyday phenomenon, read on below.
How Our Eyes See Colour
Before diving into the details of why certain material colours change upon contact with water or moisture, let us first take a quick refresher on what you have learned regarding light in IP physics tuition in Singapore. When sunlight pierces the Earth’s atmosphere and lands on a grassy field, the grass looks green to our eyes because the light energy is only partially absorbed.
The leaves and other greenery absorb the wavelengths of light in the red, blue, orange, and yellow range of the electromagnetic spectrum and reflect only the wavelengths of green light. These wavelengths hit the cone cells of our retina and are translated visually into green grass.
As such, everything we see entirely depends on how they reflect or absorb light. Moreover, the object’s material composition and texture can also affect how we see colour, such as a shirt’s countless layers of tiny fibres that form a large surface area perfect for reflecting a lot of light.
And even when certain fabric materials are partially transparent, their numerous fibres still reflect colour to the observer. Take a white shirt, for example, which consists of mostly transparent individual fibres yet still generates the colour white because of their large numbers and concentrations. The interaction of all that light, air, and fabric combine to create the appearance of a solid colour in our perception.
Also, while fabrics feel smooth to the touch, they are quite rough on a microscopic level, and rough surfaces generally look brighter than smooth ones because they provide more angles for light to bounce off of, which generates more reflection and creates a brighter appearance.
Why Clothes Turn Dark with Moisture
Now that we know what happens when light strikes a dry surface, let us look at what changes when a material gets wet, specifically how the additional layer of water serves as a secondary reflective surface.
This time, let us use a bright and saturated red shirt as an example. When viewed in the light, the shirt will absorb all light wavelengths except those that appear red, which will be reflected and perceived by the eye. Once it gets soaked, the light striking the fabric must now also go through a layer of water on the fabric.
As water fills the gaps in the fibres previously occupied by air, the cloth is more likely to bend incoming light away from the observer’s eyes. This phenomenon is referred to as total internal reflection, wherein light that usually bounces back to the observer is reabsorbed by the water instead.
This reduced amount of photons of light reflecting off the fabric and into our eyes is what causes materials to turn “darker” in colour. Even though the material still reflects the same amount of light, there is now less of it being sent back to your eye.
As mentioned, fabrics tend to have a rough surface at the microscopic level, but this becomes smoothened out with the introduction of water. And since smooth surfaces have a different way of reflecting light, depending on your angle of observation in relation to the incident light and wet surface, you may be able to notice a small, bright reflection. In other words, not only does a wet piece of fabric appear darker than a dry one, but it will also appear more “shiny”.
Once the fabric begins to dry, the air reoccupies the pockets of space between its fibres. It lets the incident light bounce and reflect freely as before instead of getting absorbed or re-reflected by the water molecules.
Conclusion
Light interacts with our physical world and the human ocular anatomy in many fascinating ways, and the contents above are just a glimpse of the overlap between perspective, light, and perception vs reality. In regard to fabrics, the change in colour from being wet is not actually due to the material itself being changed, but rather its reflective properties becoming diverted or muted, hence the resulting darker shade that we see.
Whether you want to learn more about everyday occurrences or boost your performance in an upcoming exam, Tuition Physics is here to help. We offer IP and O-level physics tuition in Singapore, taught by passionate tutors dedicated to assisting students to better understand physics and reach their full potential in the subject and beyond.
