tag:aryankashyap.com,2013:/posts Aryan Kashyap 2020-01-17T10:17:52Z Aryan Kashyap tag:aryankashyap.com,2013:Post/1497206 2020-01-09T14:27:01Z 2020-01-17T10:17:52Z Why are some things darker when wet?

Have you ever wondered why some things become dark when you spill water on them? What’s exactly happening? 

Well, let’s find out!

Source

Before I explain this phenomenon to you, let’s go through some prerequisite knowledge.

  1. How we see things
  2. Types of reflection

How we see things

We see an object when light reflects off it and then enters our eye. The eye then produces a nerve impulse and sends it off to the brain to be interpreted. Light acts as a medium of information transfer here.

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This is why you can’t see things in dark; no light means that there is no way that you can channel the information about the surroundings into the person’s eye.

Types of reflection

Now let’s understand the natural phenomenon of reflection; it’s when a wave hits a surface and gets thrown back. Not absorbed (transfer it’s energy to the surface), or transmitted (go through the surface).

There are two types of reflection (two ways the wave can be thrown back).

  1. Specular
  2. Diffuse

Specular Reflection

This is when all the incident light (light which hits the surface) is reflected into a single outgoing direction.

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In this type of reflection, the angle at which light hits the surface is the same angle at which the light leaves the surface (reflected). This type of reflection tends to happen on smooth surfaces like a mirror or glass.

Diffuse Reflection

This is when the incident light (light which hits the surface) is scattered into all angles when reflected. This is due to the irregular nature of the surface it’s reflected off.

Source

The reflected light rays leave at a variety of angles. This is because the angle of incidence (the angle at which the light hits the surface) is different for each part of the surface.

So what’s happening?

Right, let’s say you accidentally spill water on your shirt. And now it’s dark. What’s precisely happening under the hood?

Once you spill water on the shirt, that part of the shirt is now covered with a thin film of water. So, any light which has to reflect off that part of the shirt has to go through water. 

Before water is spilt, 100% of the light travelling towards that part of the shirt will hit the surface. But now only a fraction of the light moving towards it will hit its surface. This is because the light now has a layer of water to go through. And due to the reflectance of water, not all light at the air-liquid-interface (border between air and water) goes through the water. Some of it is reflected.

The probability of light getting transmitted into the water is (1 — Rl).

Rl — reflectance at the air-liquid interface

Reflectance is a measure of the proportion of incident light, which is reflected. Every surface tends to have a reflectance: how much it reflects light. So now, we’ve already lost a proportion of the total light, which was supposed to reflect on the shirt and go into the person’s eye.

Now, the incident light ray is inside the water. And it then hits the surface of the shirt. Some of the light is absorbed there, and the rest is reflected diffusely. How diffuse reflection occurs is explained above.

Due to the diffuse reflection, the light ray is scattered in many directions. All the light rays are now heading to the liquid-air interface, which is the border between the liquid and the air. Also, some of the light is sent out in a way, that it can be internally reflected at the liquid-air interface. 

Let's understand the phenomenon of total internal reflection.

Total internal reflection is a natural phenomenon, where the incident light is reflected at media interface.

It only takes place if:

  • The current medium is denser than the medium it is going into.
  • The angle of incidence is greater than the so-called critical angle.

Source

Since water is denser than air, and the reflection is diffuse. A lot of light is internally reflected, thereof, increasing the probability of absorption at surface.

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All of the phenomenons stated above come together. Due to which, the intensity of light which the observer receives is way less, relative to if the shirt was dry. This is why the shirt looks darker when wet.

So this is it!

Everything in this essay comes from this source

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Aryan Kashyap