As the saying goes, "the eye is the window of the heart". We use it to appreciate the changes of the four seasons, read the vast sea of books, and put the whole world in our mind.

However, excessive use will eventually make it overburdened, gradually become blurred, and finally rely on glasses to make it clear and bright again.

The incidence rate of juvenile myopia in China is 50%-60%, ranking the highest in the world. The total number of spectacles is about 800 million.

Both the nearsighted glasses that enable students to see the blackboard clearly and the reading glasses that help the old man read the newspaper have a common feature: they will reflect specific colors under the fluorescent lamp or sunlight, such as emerald green:

Fluorescent lamp reflected by glasses (photo source: taken by the author)

Or the blue of the sky:

How do these colors come into being? Why do different glasses reflect different colors? Next, we will use the knowledge of optics to solve this problem.

Why do glasses have color

First of all, we should start with the material of the lens. Compared with glass lens, resin lens is used by more and more people because of its light weight and non fragile characteristics.

The biggest disadvantage of resin lens is that the light transmittance is obviously worse than that of glass lens, so that the intensity of light incident into human eyes through the lens is significantly reduced, and the intensity of light reflected back is increased, so that the object we see will be darker than its actual appearance.

When others look at us with glasses, they will see the strong reflected light on the lens, just like the light curtain on the top. They can't see the bright eyes hidden behind the glasses, and they can't take pictures clearly.

In order to solve this problem, coating technology is applied to the lens to reduce the reflection of light. In this way, the light through the lens will be significantly enhanced to achieve the purpose of both beautifying yourself and brightening others.

The working principle of antireflection film (or antireflection film) is shown in the figure. In short, it is the purpose of using the reflected light formed by the incident light on both sides of the antireflection film to interfere with each other to form interference cancellation, so as to minimize the reflected light intensity and maximize the incident light intensity. Working principle diagram of antireflection film (picture source: made by the author)

The thickness of the antireflective film is directly related to the wavelength (or color) of the light to be antireflective. If the color of the light to be antireflective is different, the thickness of the antireflective film is different. Therefore, a single antireflective film cannot achieve the same amount of antireflection for all colors of light.

In this way, under the irradiation of white light formed by red, orange, yellow, green, blue, indigo and purple, such as sunlight or fluorescent lamp, some light will be reflected because there is no or insufficient antireflection, which explains why different lenses reflect different colors.