Invisibility is generally thought of as magical, unobtainable, something out of Harry Potter or a superhero comic. But optics, the physics of light, is very much science. How can you manipulate light so that you are completely invisible?
We see an object when light hits or goes through it and comes back to our eyes. Therefore, then, there are two things we can see: reflection, the bouncing of light, and refraction, the bending of light. Because of this, we can see glass and water even though they are transparent.
Latias, then, bends the light around its body to appear invisible. When light refracts, the angle that it bends is proportional to the index of refraction (n) of the material. Snell’s Law says:
Don’t get too discouraged by the math: it just means that the larger the index of refraction is, the more light will be bent when it enters the medium. For example, air has an index of refraction close to 1, and diamond is 2.4.
This also means that if two mediums have the same index of refraction, then light won’t bend at all when it passes through it. For example, this glass rod has the same index of refraction as the oil, which makes it disappear:
That’s invisibility, but not the kind we’re looking for. If Latias’ feather had the same index of refraction as the air that it flies through, then yes, its feathers would be invisible. But you could see straight through to her skin, organs, lungs, heart, eyes, etc. We aren’t looking for a way to make light go through Latias, we want the light to go completely around her.
Until recently, this was largely thought impossible. To bend light completely around a visible object like an invisibility cloak, you would need a negative index of refraction. By definition a substance’s index of refraction is calculated with a square root, and we can’t take a square root of a negative number. There was no substance on Earth that met these requirements.
Until, in 1964, Victor Vesalago mathematically proved that a negative index was not impossible. You simply need to take a “left hand rule” approach instead of the standard “right hand rule” for electromagnetics. And much more recently, in 2000, we’ve successfully engineered materials with negative indices of refraction.
The catch is, light is made up of a spectrum of different wavelengths. The “invisibility” material that we have only cloaks the object from microwaves, not visible light. Research is still being done, as you can imagine, true invisibility would be a revolutionizing breakthrough.
Latias’ feathers have a negative index of refraction, bending visible light completely around her body and making her invisible.
Because she’s not invisible all the time, Latias must have several layers of feathers; the red and white for normal, and the invisibility ones for stealth.