Why does a watermark appear lighter when paper is held up to light?

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paper mill — papermaking Paper mill in British Columbia, Canada.

A watermark appears lighter when held up to a light source because it is created by a variation in the thickness of the paper fibre during the wet-paper phase of papermaking.

Specifically, a standard watermark is formed as a thinner section of the sheet. Because this area contains fewer fibres, it allows for greater transmission of light compared to the surrounding, thicker parts of the paper, making the design appear lighter when illuminated from behind.12