What color is a mirror?

The mirror as we understand it today has its origin about 200 years ago in Germany. Undoubtedly, they are part of our life in more ways than we can imagine because of how accustomed we are to them.

But, despite this, surely there is a question that you have ever asked yourself. And if all objects have one or more colors associated with them, what color is a mirror? Perhaps, the most logical answer seems to be that of "it has no color", as it simply reflects light, but the truth is that they do have: they are slightly green.

It is true that mirrors are, in reality, the color of what they reflect, but the science behind color and these mirrors gives much more. And immersing ourselves in a journey through the nature of color in mirrors will be, as you will see, fascinating.

In today's article, in addition to understand exactly what is the physics behind colors and light, we will discuss why mirrors are, surprising as the statement may seem, green in color. Let's go there.

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    Electromagnetic waves, light and color: who is who?

    Before getting into the topic of mirrors, it is extremely important (and interesting) that we understand the science behind the color of objects. And for this, we must talk about three key concepts: electromagnetic waves, light and color. So let's see who is who.

    1. A Universe of electromagnetic radiation

    All matter is composed of atoms and subatomic particles in constant motion (except at absolute zero temperature, which is -273.15 ° C) which will be higher or lower depending on its internal energy. And as a result of this energy, there will be a temperature. Therefore, the higher the particle movement, the higher the temperature.

    And in this sense, all bodies with associated matter and temperature (which is, in essence, all the baryonic matter in the Universe) emit some form of electromagnetic radiation. Absolutely all bodies (and we include ourselves) emit waves into space that propagate through it. And depending on the energy of the body, these waves will be more or less narrow. And here we start to link things.

    A very energetic body emits waves of very high frequency and very low wavelength (the crests of each wave are very close together), while a low energy body emits waves of very low frequency and very high wavelength (the crests of each wave they are far apart). And this allows the waves to be ordered in what is known as the electromagnetic radiation spectrum.

    In the electromagnetic spectrum, the different waves are ordered depending on their wavelength. On the left we have those of high length (and low frequency), which are the least energetic: radio waves, microwaves and infrared (the one emitted by our body). And on the right we have those of low length (and high frequency), which are the most energetic and, therefore, dangerous (potentially carcinogenic), such as ultraviolet light, X-rays and gamma rays.

    Be that as it may, the important thing is that both those on the left and those on the right have one characteristic in common: they are waves that cannot be assimilated for our sense of sight. That is, they cannot be seen. But right in the middle of the spectrum the magic happens: we have the visible spectrum.

    2. The visible spectrum and light

    The radiations of the visible spectrum are waves emitted by bodies that shine with their own light (like a star or a light bulb) and that, thanks to their internal energy conditions, emit waves with just the right wavelength to be perceptible to our eyes.

    The visible spectrum ranges from wavelengths of 700 nm to 400 nm. All those waves with a length within this range will be captured by our sense of sight. These waves can come either from a source that generates light or, more commonly, from an object that bounces them. And here we are already linking it with the mirrors. But let's not get ahead of ourselves.

    For now we have light waves with a length between 700 and 400 nm that, after passing through the different structures that make up our eyes, are projected onto the retina, the most posterior part of the eye. There, thanks to the presence of photoreceptors, neurons convert light information into an interpretable electrical impulse for the brain. And this is how we see.

    But is all light the same? No. And here comes the magic of color. Depending on the exact wavelength within this 700-400 nm range, our photoreceptors will get excited in one way or another, leading us to see one color or another. So let's talk about color.

    3. Where does the color of what we see come from?

    At this point, we are already clear that color is light and that light is basically an electromagnetic wave. And it is within the 700-400 nm wavelength range of the visible spectrum that all colors are in essence. Depending on the exact wavelength within this range, our eyes will perceive one color or another.

    Objects have color because they emit (if they shine with their own light) or absorb (now we will understand this) electromagnetic radiation from the visible spectrum. And depending on the wavelength, they will be perceived by our eyes as yellow, green, red, blue, violet, white, black and basically the more than 10 million shades that the sense of sight can capture.

    Red corresponds to 700n, yellow to 600nm, blue to 500nm and violet to 400nm, approximately. The origin of the color of objects that shine with their own light is very simple: they have that color because they emit waves with the own wavelength of that color. But this is not what interests us. What interests us today, when talking about mirrors, are those objects that do not emit their own light, but rather reflect and absorb it.

    Visible light emitted by a body that does shine is reflected on the surface of such objects (including mirrors). We see them because the light falls on them and bounces back to our eyes, allowing us to capture the light. And it is precisely in this "bounce" that there is the magic of color.

    We see the color that the object is not able to absorb. We see the wavelength that has been reflected towards our eyes. If a soda can is green, it is green because it is capable of absorbing the entire visible spectrum except the wavelengths of green, which is about 550 nm (between yellow and blue).

    And, importantly, an object is white when it reflects all wavelengths. White, then, is the sum of the entire visible spectrum. All the light is reflected towards our eyes. And instead, an object is black when it absorbs all wavelengths. Black is the absence of light. No radiation in the visible spectrum is reflected. And this is, in essence, the science behind color. Now we are more than ready to finally talk about mirrors.

    Why are the mirrors green?

    If you have just read the last point above, surely a question has come to your head: if mirrors reflect all the light that falls on them, why are they not white? What is the difference between a mirror and a white t-shirt? Basically, the way they reflect light.

    While a white T-shirt and any other object (except those with mirror properties) experience diffuse reflection (light is reflected in many directions), mirrors undergo specular reflection.

    That is, in mirrors, the reflection does not occur diffusely (which is what makes, in the end, everything is combined in a single white color by union of all the wavelengths), but the light, when impact and come out bounced, due to the physical properties of the mirror, it is organized without losing the configuration with which it arrived.

    That is, in a mirror, the wavelengths are not reflected in a scattered way, but rather at the same angle at which they arrived. Specular reflection allows a reconstructed image of the object in front of the mirror surface to reach our eyes.

    Therefore, mirrors can be understood as "a white that does not mix" thanks to their physical structure and chemical composition. Mirrors consist of a thin layer of silver or aluminum that is deposited on a silicon, sodium and calcium glass plate that protects the metal.

    And it is precisely this mixture of materials that explains that, although they are technically "white", since they reflect all the light that falls on them, they are, in fact, slightly green. Silver, silicon, sodium and calcium give the mirror chemical properties that they cause that, even slightly, it has a tendency to absorb less the wavelengths of green, which we have already said are approximately between 495 and 570 nm.

    In other words, mirrors reflect green better than other colors, so they are slightly green. This can only be perceived in the infinite mirrors, where we see that the image, with infinite reflections on itself, becomes increasingly green, as it reflects more and more light of this wavelength typical of the color green. No mirror reflects 100% of the light that falls on it. Therefore, it is natural that there is a color (green) that reflects better than others that absorbs more.

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