Why do papers turn clear when touched by a liquid?

Answer 1

Paper is composed of many small fibres which are separated by air.

When a beam of light passes through the fibres, it gets highly refracted by the air.

The difference in refractive indices of the fibre and air lead to greater change in the angle of refracted light. As a result, most of the refracted light beams subsequently fall on fibres and is scattered off the paper in a more or less random direction. The amount of reflected light is great and hence we see a bright white paper.

Paper has two sides. Let's call them 1 and 2.

From side 1, light passes into the paper but a lot of rays are reflected and only a few actually pass through the paper. When you are looking at the paper from side 2, you cannot distinguish the small amount of refracted light coming from side 1 through the paper because at side 2 also, light will try to pass through and will largely get reflected. Thus you cannot see the light from the objects behind the paper effectively.

When the paper is wet with water (a liquid), the air between the fibres is replaced by water. Water has a higher index of refraction than air and so the relative index of refraction between water and the fibre is much lower than between air and the fibre. As light passes through the water, it is bent less and mostly hits another water molecule. This way, it will travel through the water in the paper and passes out of the other side. And each time light passes through, it gets dimmer. So it will be a lot dimmer when it leaves the paper. Even if the intensity is low, the amount of light passing through the paper will be large. Also, the surface tension of water molecules bind the loose fibres and thus reflection is reduced.

From side 1, the light is passing through. Most pass through the paper and very little is reflected back. Thus you see a dark, wet paper. The same happens on side 2 and a large amount of light passes through to the side 1. When you look at the paper from side 1, you can see what's behind the paper as the light rays reflected off them reach your eyes through the paper. Since both sides are wet, they are similar in behaviour.

Additional info: Through a wet paper, you may see only the writings on the back of the paper or objects very immediately behind it. This is because light rays from farther objects reaching the paper may not be intense enough after passing through the wet paper.

When paper comes in contact with oil (a liquid), the same happens. But the relative index of refraction between oil and the fibre will be very much less. Thus, most light rays pass through and the paper becomes translucent.

Answer 2

Paper turns clear when touched by a liquid because the liquid dissolves the fibers in the paper, causing them to spread apart and allow light to pass through. This effect is due to the refractive index of the liquid being closer to that of the paper fibers, making them less visible.

Answer 3

Paper does NOT react with water, it even doesn't dissolve in it. Only absorption (wet paper)