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Butterflies also understand physics and use the principle of thin-film interference to make blue.

2025-01-14 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >

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Animals in nature have a variety of colors, but only blue is the rarest. Butterflies, for example, basically have the most complete colors and use these colors to convey messages, such as "I am poisonous" or "I am a male". If we zoom in on the butterfly's wings, we will see that these colors come from tiny scales. These scales contain pigment molecules that absorb light from other colors and reflect light from the colors we see. For example, red pigment molecules reflect red light and absorb light of all other colors.

But the color of butterfly wings is not all this principle, there is a kind of butterfly called the Great Blue Butterfly, the blue of its wings is somewhat special, its scales actually do not have blue pigment molecules, blue mainly comes from the arrangement of the scales themselves. This color is called structural color, and the structural color of an organism is produced by the physical interaction between light waves and biological nanostructures with different refractive index. If we look at the blue butterfly from different angles, we will find that its blue is changing, just like a hologram. By the way, only the great blue butterfly of the male has a beautiful blue.

The structural color of the blue butterfly is attributed to a variety of physical mechanisms, including multiple interference, diffraction, Bragg scattering, Tyndall scattering and Rayleigh scattering. In this article, we will talk about the main intervention mechanism. However, before we do that, we need to briefly review the thin film interference.

Thin film interference assumes that a beam of light irradiates the film, which will reflect on the upper surface of the film, and it will also refract into the lower surface of the film, and then be reflected by the lower surface. Finally, the two beams of reflected light interfere to form new light waves. Here, we should pay attention to the half-wave loss when entering the optically dense medium from the optical sparse medium. Therefore, we can calculate the optical path difference of the two reflected light beams. If the optical path difference is odd times of the half wavelength, the phase length interference occurs; if it is even times, the cancellation interference occurs.

Thin film interference is widely used in real life, and one of our most common applications should be antireflective films. We hope that during the shooting process, the light of unpopular colors can enter the camera less, while the light of other colors can enter the camera more, then the antireflective film will come in handy, through the special refractive index and thickness design, it can let unwanted light interfere with each other, and other light can enter the camera more. The color in front of the camera lens is the color of the reflected light.

Structural color so, what does the blue of the big blue butterfly have to do with all this? If we enlarge the scales on the wings, we will see some small ridge shapes. As we continue to zoom in on these ridges, we will see that it is shaped like a small Christmas tree, and the arrangement of "branches" makes its wings blue. When the light enters the structure, part of the light will be reflected on the upper surface of the "branch", while the other part of the light will enter and reflect on the lower surface. As a result, just like the film interference, the two beams of reflected light will interfere.

The wavelength range of blue light is from 400 nm to 480 nm, and the optical path difference caused by the scale of the structure is about 200 nm, which is exactly half the wavelength of blue light. While other wavelengths of light, canceling interference occurs. In addition, the scales are attached to the substrate containing melanin, which absorbs stray light such as red and yellow, which highlights the blue.

If we fill these tiny gaps with something else (such as alcohol), then the blue will disappear. This is because it changes the refractive index, which changes their optical path difference. So, you might ask, why doesn't it change color when it rains? Because these scales are made of natural waterproof materials, water cannot fill these gaps.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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