What are the optical properties that cause the glossy look of wood varnish? Applying varnish to a painting or wood makes the colors more vibrant. Why?
 A: Disclaimer: Since I am not an expert in this field, this is in no way a complete answer. The only intention is to provide some insights into some of the notable developments in the area, from which the readers can set out to explore.
What an interesting observation! Wooden surfaces offer an incredibly uneven surface for the reflection of light. When you apply varnish on its surface, it would seep into all the irregularities and solidify into a smooth outer skin. Most of the reflections from this surface will be direct reflections rather than diffusive reflections,  resulting in glossiness. Apart from the glossiness, making the surface more reflective can give it a brighter appearance.
But that's not all; applying varnish to an object can change its color. The effect is due to the changes in reflective and scattering properties of the  varnish layer and the its pigments.
For many artists, varnishing is an essential finishing touch for their artworks as it brings out the fine details in them. It is also used in art resoration projects.
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