Also, why it is always yellow color, and not any other color?
In: Chemistry
Different materials with different colors reflect different parts of the rainbow. What makes things white is when they reflect all colors of the rainbow.
This means they reflect “low energy light” near the red end of the rainbow” and they reflect “high energy” light near the blue and violet end of the rainbow.
High energy light is called high energy because it is able to do you things like break chemical bonds easily. Once those bonds are broken they don’t reflect that color light as well anymore.
So over time the higher energy colors like blue and violet and green get broken more often and this pushes the color to lean a little bit more towards yellow or red. In paper, the chemicals that tend to reflect these colors are more easily broken and certain kinds of paper end up looking more yellow.
Not all paper does. Paper made from cotton for example, commonly used for watercolour painting doesn’t yellow, at least not in the same way.
Most modern paper is made from wood pulp which contains lignin. This lignin is what turns yellow over time. Paper is bleached to make it more white but that’s not the natural colour.
Paper contains a chemical called lignin. Sunlight and air causes this molecule to break down over time. This change in structure results in it absorbing more like, thus appearing darker and making the paper yellow and brown.
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