When you spill water on your shirt, it looks darker than the rest of the cloth.
Why does it happen?
The lack of awareness regarding this question still blows my mind. This phenomenon is so regular that most of us ignore it as if it’s a universal truth of nature. You don’t question, others don’t answer.
This is what we experience daily without giving a thought:
If water spills on your shirt → Wet area gets darker
You might ask why this kind of unawareness among people blows my mind. People are unaware of lots of things that go unnoticed.
Answering to that: This phenomenon lets people notice when you pee your pants, causing a load of embarrassment still, you don’t know the mechanism behind it.
Isn’t that mindblowing?
It’s not just your shirt or your pants that show this behaviour. All of the so-called daily life materials become darker when they get wet.
If something gets wet, it is bound to get darker. This fact is “usual” to that extent, nobody questions it.
Getting to the physics behind it...
Let’s assume that you are wearing a blue shirt. What makes the person standing in front of you know that the shirt you’re wearing is blue?
It is done by the absorption of electromagnetic waves by the material of your shirt.
Radiation from a source, say sunlight, strikes on your shirt. This electromagnetic radiation is a set of distinct wavelengths. Visible light is a small part of the entire set, which is known as the Electromagnetic Spectrum.
After striking on your shirt, waves with wavelengths not lying in the region of 380 nm - 500 nm, from the visible light part, are absorbed by the material of your shirt. The remaining waves (of wavelength lying in 380 nm - 500 nm) get back to your eyes. These waves are detected as blue… by your eyes.
But that’s not the case when water is spilt over your shirt. Light has to travel through a completely different journey.
Light strikes on the surface of the water and undergoes Refraction (because the refractive index of water is greater than that of air). On travelling further, it undergoes Reflection after striking on your shirt. Some part of the light is absorbed and the remaining light (with the wavelength of the blue region) goes further.
On reaching the boundary of water, if the angle of incidence is greater than the critical angle, light undergoes Total Internal Reflection. So, it bounces back to the water medium.
[The value of the critical angle is determined by the Arcsine of the ratio of refractive indices of the two corresponding mediums — here, air and water]
We know that the intensity of light decreases when it undergoes absorption. So, when light strikes on the surface of your shirt, again, its intensity is reduced.
On bouncing back to the surface of the water, it refracts further (if the angle of incidence of the light ray is lesser than the critical angle), and reaches your eyes. Your eyes interpret this as the darker version of blue.
So, the next time your friend scares the sh*t out of you and you pee your pants, curse total internal reflection.
Source: Sunny Dhondkar, Quora.com
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