If you were among the asocial few who actually listened to their Physics teacher, you'll probably know most of what follows. But maybe you were cool instead, and even if not, surely some of the details go beyond what you know. In any case, this is one more post (after this one) on the way of explaining what I did during my PhD. Enjoy.
What gives objects color?
What gives objects color?
Color, of course, has everything to do with light. Now, light is actually electromagnetic radiation, and as such propagates in waves, and every wave has an associated wavelength. Classically, this is the distance between two consecutive crests. Quantum mechanically, the wavelength also determines the smallest amount of energy that a beam of light can carry - namely, the energy of one light 'quantum', commonly called a photon. The higher the wavelength, the lower the photon energy. I’ll get back to this later on. Importantly, individual wavelengths, or photon energies, are also associated to various colors. Or, rather, the other way round - to every color, a particular wavelength can be associated, but not every wavelength has an associated color. This is simply due to the limitation of our perception - we cannot register all the wavelengths, and the concept of color is only associated to the ones we do. You should be familiar with some version of the following image, which illustrates the full range of possible wavelengths, and what we call the corresponding radiation:
|"Electromagnetic-Spectrum" by Victor Blacus. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Electromagnetic-Spectrum.svg#/media/File:Electromagnetic-Spectrum.svg|
Now, the range of wavelengths that our eyes detect is not at all arbitrary. This is what the spectrum associated to sunlight reaching the Earth looks like:
|"Solar spectrum en" by Nick84. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg#/media/File:Solar_spectrum_en.svg|
Our eyes have naturally evolved to detect light in the range in which there is the most of it! If it’s not clear why, I’d have to explain natural selection to you, and that’s well beyond this post. But sunlight brings me to the next point: so far we only discussed color in relation to the spectrum of a beam of light. With this in mind, the color of objects is naturally determined by the light coming from them. However, since most objects around us don’t emit light on their own - at least not in the visible range - color is implicitly also determined by the spectrum of the light illuminating them. In other words, if we were to illuminate any object with monochromatic blue light, it would appear either blue or black, or something in between - because no other hue can mix up in the spectrum. Thus, in a sense, we should call the 'true' color of an object the one we see after illumination with light of equal intensity at all wavelengths, otherwise known as white light. Sunlight is to a good approximation white light, see image above.
Finally, I come to the main point. What determines the color of an object once we fix the observer (an average human) and we fix the illumination (white light)? Both of these factors are important, but they are extrinsic to the object itself. There must also be some intrinsic property that gives an object its hue. Indeed, its physical and/or chemical structure determine its optical features, and more specifically - the absorption, reflection, and transmission properties. These are related to a wide variety of phenomena and can be incredibly complex to study and predict, but here we take them for granted for illustrative purposes. The important point is that there are several possible outcomes for light hitting an object, and the interplay between those determines its color. Namely, light can be transmitted, reflected, scattered, or absorbed, and normally all of those happen simultaneously - to a various extent.
this Vsauce video for further insight into the color of mirrors.
Finally, when there is mostly transmission, the object appears translucent because we see the light coming from behind rather than the illuminating light.
These qualitative consideration give a lot of intuition about color. So, while we are at it, why not answer the age-old mystery: why is the sky blue? In fact, it's not really - or rather, it can take on various colors, blue being just one of them. The first important note is that the sky is actually mostly translucent (assuming no clouds). When you look at the Sun, you see it (right?), and when you look back at Earth from space, you see it. Furthermore, the Sun looks white (technically kinda yellow-ish because the spectrum is not exactly white light), and around the Sun the sky looks white, too. It doesn't look blue, because we're mostly seeing the light coming directly from our star.
language shapes the way we perceive colors. Blue is a particularly weird color, and believe it or not there's evidence that you might not know that the sky was blue had you not heard that all your life. But, at least when it comes to Physics, you now know why it should be somewhat blue-ish. Apart from the cases when it's white-ish or red-ish...