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Original title: "what color is the mirror?" 99% of the people answered incorrectly! "

Who am I? Where am I? Why would I ask that question? Photo Source: pixabay many people look in the mirror every day, but there are different opinions about the color of such a common and commonly used object. Silver, white with green, black. What is the scientific basis for these very different statements?

If you treat a mirror as an ordinary object, an interesting question arises: what color is the mirror?

Some people may think that the mirror has no color, and it is what you use it to shine on. Some people think that mirrors are just silver. You can say that in life, but to be serious, there are problems with both answers. In order to explore "what color is the mirror", we first need to understand what the color is and where the color of the mirror comes from.

Everything we see from light to color is because the light it emits enters our eyes, projects it on the retina through the refraction of the eyeball structure, and the light signals are converted into electrical signals by the photoreceptor cells, which are transmitted to the brain through the nerves. Finally, the image is processed by the visual center of the brain. The different colors we see actually correspond to different combinations of visible light at various wavelengths.

The cones in the retina are responsible for identifying different light wavelengths. Most people have three different types of cones, which are sensitive to blue light with shorter wavelength, green light with medium wavelength and red light with longer wavelength. The combination of three kinds of signals of different intensity is processed into rich colors by the brain.

Electromagnetic spectrum (photo source: Ufoismey3k / Wikipedia) however, this is the way humans "see" colors, not "understand" colors. When you see the beautiful landscape, you don't need to think about "how much blue light, how much green light, how much red light", but "the distant mountains are grayish blue, the rivers are dark green" and so on.

Such expressions reflect the three dimensions of our understanding of color: hue (hue), saturation (saturation), and brightness (value). The difference in hue, such as blue and green, corresponds to the wavelength of the main light; the difference in saturation, such as blue and gray blue, corresponds to the complexity of light; the difference in lightness, such as light green and dark green, corresponds to the amplitude of the light wave.

HSV color model. Different angles represent different hue, the saturation increases from the inside to the outside, and the brightness increases from the bottom to the top. (photo source: SharkD / Wikimedia Commons) you may have noticed that there is no white light in the visible spectrum of red, orange, yellow, green, cyan, blue and purple, so white is not a hue, but white can be a color. When a variety of colors are mixed with enough intensity, a color with no prominent hue, low saturation and high brightness is formed, which is white in the eyes of our brains. Sunlight is a kind of white light, which contains all wavelengths of light in the visible spectrum.

When an object is illuminated by white light, it will absorb part of the wavelength of light and reflect part of the wavelength of light. the color of these reflected light is what we usually call the color of the object. Why emphasize "being illuminated by white light"?

Imagine an extreme situation: a blue car that absorbs all wavelengths of light except blue. If the only street lamp in the dark just emits red light, it will be completely absorbed by the body, making the car look black. It can be seen that color is not an inherent attribute of an object. But unless we deliberately lift the bar, we will still say, "there is a blue car parked there," because what we usually call the color of the object is for white light.

Still remember the blue-black skirt or the platinum skirt dispute that caused a sensation? It is unrealistic to talk about the color of an object without light. When people ask about the "original color" of the skirt, they are actually asking what color the dress will look like in white light. (photo source: Wikipedia) back to the question of "what color is the mirror?" Common glass aluminized or silver-plated mirrors can reflect almost all light. When white light shines on the mirror, the reflected light is naturally white. Therefore, we can say that the mirror is white. "what you look in the mirror is what it is" is not a good answer because it ignores that we usually discuss the color of objects under the condition of white light.

However, the answer to "white" is not perfect. The reflected light of the metal aluminum or silver used to make the mirror is not particularly biased, but don't forget that there is a layer of glass in front of the metal, while the commonly used silicon glass has the weakest ability to absorb green light. A 2004 physical study found that the maximum reflection spectrum of mirrors ranges from 495 to 570 nanometers, equivalent to green. In other words, the mirror is white with a little bit of green.

Usually this green tone is difficult to detect, but it becomes obvious when we place the two mirrors opposite each other and let the light reflect each other constantly. This device is called "mirror tunnel" or "infinite mirror". The deeper the tunnel in the mirror, the darker and greener the color is.

Relative mirrors are very common in daily life. The next time you encounter them, you can pay attention to the depth of the mirror tunnel. Is it dark and green? (photo Source: editor himself) I believe there must be some people who cannot accept the answer that the mirror is white (green). (actually, I can't) you may be thinking: a mirror looks completely different from a white wall or a piece of white paper, isn't it silver?

Let's first see what "silver" is. There is the name "silver" in some standard color tables, such as # c0c0c0 in the Web color table:

Is it different from the silver you think, it just feels like an ordinary gray? How about this:

Is it a little silver now? But I didn't change the color value, I just added a gradient effect. It can be seen that what we call "silver" in life is not a real "color", but a gray-white state with luster. The same is true of metal colors such as gold.

We learned in junior high school physics class that the reflection of an object to light is divided into diffuse reflection and specular reflection, both of which conform to the law of reflection. When diffuse reflection occurs, the incident light changes direction in the rough surface or internal structure of the object, and the direction of the reflected light is uncertain. The reflection on the surface of white paper and white wall belongs to diffuse reflection. When specular reflection occurs, the incident light is reflected regularly, and the reflected light direction of parallel light is still parallel.

Diffuse (left) and specular (right). (photo Source: MikeRun / Wikimedia Commons) Mirror is an extreme example, because the aluminum or silver coated on the glass surface is so smooth that almost all the light is reflected by the mirror, so a clear image can be obtained. Think about the process of polishing the bronze mirror, so that the metal becomes flatter and more specular reflection occurs on the surface, so that the effect of light can be seen. The glossiness of a metal is caused by the specular reflection that occurs on its surface.

Therefore, the difference in visual effects between "white" mirrors and white paper and white walls is only because of the different modes of reflection. The "silver" of the mirror is not a special "color" different from white, but refers to the metallic gloss caused by specular reflection.

Why does the metal surface produce a particularly shiny luster? This is a complex problem, which, if explained roughly, has something to do with the microstructure of the metal. We learned in chemistry class that the metal element is made up of atoms, and the electrons farthest from the nucleus tend to break away from the nucleus and become free electrons. Free electrons form a negatively charged plasma (plasma) near the metal surface, which has the ability to move freely and is a mass of free electron gas. The electromagnetic wave can change the distribution of the charge.

Metal bonds, red for negatively charged free electrons, blue for positively charged metal cations. (photo source: Julen Aduriz EHU / Wikimedia Commons) visible light, as an electromagnetic wave, oscillates the plasma made of electrons when it hits the metal surface. As a result, the plasma produces another electromagnetic wave opposite to the incident electromagnetic wave, that is, reflected light. The energy loss in this process is small, so the reflected light of the metal can be very bright.

If the frequency of the incident light is relatively low, such as visible and infrared light, the speed of the electron can match it, and the incident light can be reflected by the metal; but if the frequency of the incident light is too high (such as ultraviolet light), the electron can not match its speed, the metal will be penetrated by light. Different kinds of metals have different abilities to reflect light. For example, silver is transparent to ultraviolet light with a wavelength less than 140nm, but it has a good reflection ability in the visible band, so it is suitable to be used as a mirror.

Color and luster at this point, there should be no dispute that the mirror is basically white, but there is another "violence theory" worth mentioning: the perfect mirror is black. Why is there such a saying?

You can look up and look at the shiny objects around you, such as ceramic cups, plastic bags, and even some glossy fruit. As an example of the apple in the picture below, it is clear that the white spot is the specular reflection of the light source on the apple surface, while the red part is diffuse. According to the previous definition, the reflected light of an object when illuminated by white light is the color of the object, so if you were asked to describe the color of the apple, would you say specular white or diffuse red?

Obvious specular reflection can be observed in waxed apples (photo source: Pixabay). It is believed that most people will not think that apples are white just because they have reflections. In other words, the "color" and "gloss" of an object need to be discussed separately, in which the diffuse part can be classified as "color" and the specular part as "luster". Although the two are often confused in daily life, color and gloss often need to be described separately in areas such as the paint industry, which have higher requirements for visual effects.

And gloss will affect the color, which is easy to understand. When the amount of incident light is constant, more parts are used for specular reflection and less for diffuse reflection. This is why a smooth surface usually looks darker than a matte surface for the same material.

So what color is the mirror? The conventional answer is "white (green)", and the answer to distinguish between color and gloss is "dark gray (green)". If you don't stress words in life, "silver" with both color and luster is also a good answer.

The color of the mirror ultimately depends on your definition of what the color of the object is, and you don't have to pursue the only answer. By discussing the mirror, which is a bit strange, it may be a more important harvest for us to better understand how human beings observe, recognize and describe colors.

Reference link:

Https://www.zmescience.com/science/physics/what-color-is-a-mirror-its-not-a-trick-question/

Https://aapt.scitation.org/doi/abs/10.1119/1.1615524

Https://engineering.mit.edu/engage/ask-an-engineer/why-doesnt-a-plain-white-piece-of-paper-reflect-light-but-a-mirror-does/

Https://en.wikipedia.org/wiki/HSL_and_HSV

Https://www.quora.com/Why-are-metals-shiny

Https://scienceline.ucsb.edu/getkey.php?key=3903

Https://www.zhihu.com/question/24362870

Https://www.quora.com/What-is-the-colour-of-a-mirror

Https://www.iiconservation.org/sites/default/files/news/attachments/7820-iic-itcc_2016_lecture_david_saunders_colour_and_gloss.pdf

Https://www.wonderfulcolors.org/blog/how-gloss-effects-paint-color-perception/

Https://sensing.konicaminolta.asia/understanding-the-effects-of-gloss-in-color-measurement/

Https://www.qualitymag.com/articles/91148-color-vs-appearance

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Zheng Yuhong, revised by Wang Yu

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