Why do coated lenses look purple (or blue or green or whatever)?
First, let’s get the most obvious misconception out of the way:
Lens coating has NO COLOR. It is a water-clear material throughout the visible spectrum. It has only 2 meaningful characteristics that make it appear colored: (1) Refractive Index, and (2) Thickness.
To understand why coated lenses look colored, you have to understand what the coating is there for, and how it works. The purpose of coating on a lens is to reduce reflections from the lens’ surface. Light reflects from the surface of the glass because the refractive index of the glass is different from that of the air, and the reflection is formed at this interface of different refractive indices.
How does lens coating work
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How does lens coating work
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