Updated: May 6, 2022
Don't you hate it when people who think they're smart ask dumb questions like, "What happens when an irresistible force meets an immovable object?"
I've had to endure many dumb questions. One of them I've hated the most is one I heard when I was six-years-old. I detested it even back then. Here it is:
“If a tree falls in a forest and no one is around to hear it,
does it make a sound?”
I despised anyone who asked the stupid question because I knew sound came from things hitting each other, whether anyone was there to hear it or not.
Then, many years later I heard a question that actually sounded interesting to me:
What is the color of unexposed film?
I looked into it. Unfortunately, the answer is anticlimactic:
The color of unexposed film is
the same color as exposed film.
Unexposed film does not change color when exposed to light. Rather, exposure to light alters chemicals within the film, which later become visible when the film is developed.
Moving forward to the present day, let’s ask a more exciting question:
What is the color of an apple
when placed in complete darkness?
The answer to this question will surprise you! Get ready for it.
A red apple in complete darkness is...invisible.
I don’t mean it’s invisible because it’s in darkness and we can’t see it. I mean that it is literally invisible.
All objects we see are not the objects themselves, but light reflected from those objects. A red apple isn’t red because it’s red, but because it reflects red light.
To clarify the question a bit, what is the color of the apple core inside the apple? The outside of the apple is red. But what about its core, beyond light’s reach? The answer is it's invisible.
Stay with me here. I can offer something of a proof. Wouldn’t it be nice to see an apple with a special, magical light that doesn’t reflect back so we can see the apple in its natural invisible state instead of tarnished by that pesky reflected light?
Wouldn’t that be neato?
It turns out that there is such magical light. It’s called X-ray emission. X-rays pass through things so we can see their insides, as if the objects were invisible.
There it is: if light passed through something, it would look invisible. Luckily for us, solid objects only partially block X-rays. If that weren’t true, all X-ray charts would be blank.
I can now sense the wheels in your head turning.
Why can light pass through glass?
Why can light pass through glass but not wood, metal, or other materials? It's not because glass is soft and wispy, like smoke or air. Light can even pass clearly through diamond, which is one of the hardest materials in existence.
I have two levels of answers for you. First is the commonly-spoken-layman-answer that is easy to understand but is not really true:
Light passes through glass because the electrons in the glass
are “tied up” (busy doing other things) and thus are not available
to disrupt the photons passing through the glass.
It's true that electrons in glass and other transparent materials contain only electrons that have an “energy gap” higher than the available energy of visible light, thereby allowing the light to pass by them. However, glass refracts (bends) light and slows it down as it passes through it. This means something more complex is going on than simply light "passing through glass."
There are multiple scientific theories competing to explain how light passes through glass. They sound good, but they are very different from each other. This begs the question: Which, if any, are right? The fact that scientists disagree with each other on this matter suggests to me that no one knows--else these scientists would agree with each other.
Rather than my attempting to summarize these theories, how about watching some nifty videos (not produced by me):
The first video link mentions polaritons. There are at least six different kinds of polaritons:
Surface plasmon polaritons
You can Google those terms and get really smart, if you wish.
Now that we know that light does not simply pass through glass, we can begin to realize that when we see a tree or flower or person through a plane of glass, we aren’t actually seeing that tree or flower or person, but instead a visual “reproduction” or “copy” of that tree, flower, or person. The light entering the glass is converted into something else and then reformulated as new light on the other side of the glass. It is not the same light that entered the glass.
You’re so transparent
Have you ever been accused of being transparent and hollow? It turns out that, at least visually, you really are! Under your striking physique and alluring eyes there is nothing to see at all.
Only hollow emptiness.
As it turns out, beauty actually is skin deep.