Why is the lens of the mobile phone blue?

That's the color of the lens coating, and the scientific name is antireflection film.

When the thickness of the coating is 1/4 of the red wavelength, the reflection of the coating is blue, but the coating itself is colorless, so the color of the coating will not change (precisely, it will change slightly).

Coating is an important process in modern optics, and the quality of coating directly affects the imaging quality.

For example, the famous Datong T* (usually appearing on Sony cameras) is a patented coating process, and the lens using this coating process can be called T*.

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