Why aren't we bumping into objects outside of the visible range?

Question

Apologies if this is too simple of a question for this site. My physics education stopped at the undergrad level. My 12 year old nephew asked me a question and I couldn't give him an answer. He asked, upon learning the human eye could only see within a limited range of frequency, “Why aren't we regularly bumping into objects outside of the visible range?”

Answer 1

We do.

Birds fly into windows.

The first time I saw glasses with an anti reflection coating, I tried to poke my fingers through. I was curious why the frames had no glass in them.

You don't exactly bump into air, but you don't see it.

Cosmic rays and neutrinos fly through you all the time.

Infrared and UV photons bump into you without being seen.

Answer 2

It's a good question. but the answer is basically evolution. Organisms that frequently bumped into things that they couldn't see/detect were easy prey and so quickly removed from the gene pool.

Answer 3

Something that hasn't been mentioned yet (perhaps because it's too obvious): if an object isn't transparent, but rather doesn't reflect any light of the visible spectrum, the first instinct might be to think that it will be invisible, but actually it will just appear black to us. See: Vantablack.

(Also, as @gerrit noted, if the object is hot enough it will glow inside the visible range anyway, and all the reflection/refraction/whatever won't matter for us to see it)

Answer 4

I think your nephew is thinking something like this: Blue objects reflect blue light, and red objects reflect red light. But what about objects whose color is infrared? Why don't we bump into them?

And the answer is, we call those objects "black". Most objects are opaque to a large range of wavelengths above and below the visible spectrum. Maybe they have a color that we can't see, because they reflect strongly only at some wavelength we don't see. But, that means we just see them as black.

Many answers have pointed out that there are things we can't see, or have trouble seeing, but that requires the additional special property of transmitting the wavelengths we do see. Relatively few solid objects do that, as you know from experience.

Answer 5

As mmesser314 answered, in fact we do. When light finds the surface of a material, three different phenomena happens: transmission,absorption and reflection. Reflection is the simplest one, it is highly manifested in a mirror, for example. Transmission is what happens in a glass, and this is the reason why we see through it. Absorption is more subtle phenomena, but it is manifested when the objects are heated by light, for example. It is important to know that all the three will happen, what changes from one object to other is the amount of transmitted, reflected and absorbed light.

For an object to be invisible, in fact it should be transparent, i.e., it should transmit the majority part of incident light. The insight that is not trivial is that this in fact is a phenomenon that depends on the frequency of the incident light. Some objects could be transparent for some frequencies, but not for others.

It is not a coincidence that we can see in the range of frequency that we do. For this range, the water is transparent, for example, and it is important not only for the human eye. If some life form could see higher frequencies, more objects would be transparent (this is the reason why we use x-rays in tomography, for example).

Now, maybe you can impress your nephew telling him that the rainbow is bigger than what we can see (at least, it impressed me when I was a child).

EDIT; As pointed out in the comments, to transmit all (or the greatest part) of incident light is not enough to achieve invisibility, since we could see transparent objects due to refraction. Only a transparent object with refraction index close to the refraction index of its surroundings would be invisible to us.

Answer 6

Easy.

Glass doors are a simple example, but they are SOMEWHAT visible because of the refraction of the light. And the glass is not absolutely transparent either.

Just turn off the lights at night.

You don't see anything and you bump into object perfectly visible e.g. in 10 micrometer wavelength. You can try yourself with a thermal camera.

The ordinary matter we interact with (made of atoms and molecules) in its condensed state (liquids and solids) is "visible" in generally any wavelength from e.g. 1000m (subject to the natural resolution limits related to wavelength) down to the shortest gamma-rays we know of.

Human vision works in a rather narrow, but important range - where the spectral characteristics of an object profoundly relate to its chemical nature. This is why we see colors.

Human vision is also very, very precise in regards to angles, shapes and contours - this is how we see transparent objects (because of the refraction) and this is how we sometimes can see a heated air jet as well.

In short, there are few objects that are both solid in order to bump in them and invisible.

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This is how one can honestly bump into both solid and invisible objects:

Some plastics and some other materials are transparent and have refraction characteristics very similar to water. If you immerse them in water, they become really invisible. You can then dive there and bump into them as hard as you want without seeing what you are bumping into.

This is also how some water organisms (e.g. jellyfish) are invisible or nearly-invisible in water. They are honestly invisible to other organisms that have worse vision than humans.

Answer 7

Visible light encompasses a wide range of frequencies. So for something to be totally invisible to our eyes, it would have to be transparent in all these frequencies. Natural objects like this are extremely rare, possibly nonexistent. Air itself is transparent, but there are often substances suspended in it that are visible, such as clouds and smoke.

Glass is very transparent, but pure glass doesn't exist in nature, it's man-made. And even this isn't perfectly clear, we often see reflections and glints, and the edges of the glass are visible.

There are some other things in nature where we can't see details because they're outside our visible range. Many flowers have patterns that are only visible in the ultraviolet range that many insects can see; see A Different Light for examples.

Answer 8

I feel like some of the answers miss that this question actually works in reverse.

Why did our - and most animals' - eyes evolve to see EM radiation specifically in this range of frequencies? Because that was the most useful and practical one to focus on as a way to get information about the environment. It just so happens that it's the "just right" EM frequency range for most solid matter to interact with it in some way, and the one provided abundantly by the Sun for everything to reflect, so it was very useful. Infrared is the other one that could be even more useful, as everything emits it, and some animals do have limited infrared detection capabilities, but the downside there is that your own sensor organ would also emit infrared (since it's just a property of things at a certain temperature), so there would be a lot of noise. Good low frequency infrared cameras need to be cooled down with liquid nitrogen, that's not easy to do for a living organism.

Meanwhile, microwaves just aren't common in nature, and radio frequencies even if they were would be way too imprecise for the scales and distances living beings operate on. Lions aren't hunting plane-sized gazelles from 20 km away. On the other end of the scale, UVs aren't as abundant and the higher in frequency you go, the more dangerous things get. You could I suppose imagine some biological organism that metabolises and concentrates uranium or some other radioactive element to develop an organ that acts as a gamma emitter to use these radiation for sensory purposes. But that has several downsides. It's complicated, it's expensive, it's dangerous (high energy radiation equates cellular damage for virtually all life), and since gammas are so penetrating they are most useful in transmission, not reflection. Maybe such animals could work in packs, using each other's sources to identify what's in between them. That might be a cool idea for a speculative biology story set on an alien planet, but on Earth, light is just that much more convenient!

Consider also, now we can create glass and a few other solid things that are also transparent. Still not so perfectly non-interacting as to be invisible (the refraction gives them away), but enough to fool someone who isn't paying attention. But such things did not exist in the ancestral environment. As far as the bird that crashes into it is concerned, glass is an eldritch magical forcefield beyond its comprehension.

Answer 9

Your nephew has a really good intuition for why vision is so interesting! As several of the above answers have noted, sometimes we do bump into objects because their interactions with visible light did not render them visible to us. Typically, this is because they did not reflect (very common) or emit (less common, but happens with transparent fluorescent or phosphorescent “glowing” material) enough visible light for us to detect them. This can happen either in the case of the brightly lit sliding glass door others have pointed out, at least as long as the light doesn’t scatter in such a way that you notice it. But it can also happen in the case of objects that absorb too much light and are rendered black enough that we don’t notice them. Imagine walking down a gently lit hallway with the walls, floors, and ceilings painted with vantablack. You would not know whether to continue straight or turn because there would be no light reflecting off the walls to make it obvious if you were about to run into something. It would be a truly disorienting experience. Another special case is when there simply isn’t enough light around for us to see efficiently in the first place. I certainly remember playing games in the dark as a kid and ramming my shinbones against dark wooden tables in the dark despite being able to see some other dimly lit objects just fine. If we were to take a spacecraft into deep space far away from a source of light we would be in serious danger of collision from darkly colored objects unless we were very carefully checking for them all the time. We simply wouldn’t see them coming! The fact is that we don’t usually run into things because we have evolved to operate during daylight hours where the solid objects are sufficiently illuminated and reflective of visible light that we will not be in danger from our surroundings. Our entire definition of visible light is just the light we have evolved to see, and so it makes sense that solid objects on earth just happen to be visibly opaque to us. When we operate outside those boundaries we really do run into trouble!

Answer 10

The "visible range" refers to frequencies. Objects are not frequencies. So it doesn't make sense to refer to an object as being "outside the visible range". You may be using that as shorthand for "objects whose color is outside the visible range", but color isn't the same thing as frequency, either. Each frequency has a particular color associated with it, but that color can be achieved by lots of other combinations of frequencies. For instance, there is a frequency of light that is perceived as "yellow", but "red" and "green" light put together are also perceived as "yellow".

"Color" refers to how we, as humans, perceive combinations of frequencies, so it doesn't really make sense for there to be a color outside of the visible range. Now, you may ask "Okay, what if something has nothing but frequencies outside the visible range?" Well, first of all, that would be rather unusual. Most things reflect some light across the spectrum. Something perceived as "red" is reflecting mostly red light, but is also reflecting some green and blue light. So even if something mainly reflected UV light, it would probably reflect some light in the visible spectrum. Something that doesn't reflect any light in the visible spectrum is perceived as "black", but when we talk about "black", we generally mean something much darker than most things. Even things that are "black" still reflect some light. And even if something reflected no light at all, if it's surrounded by things that do reflect light, then you'd be able to tell that it's there because it would block light from those other things.

Another possibility is that something might be transparent. However, transparent objects generally have a refraction index that's different from air, and so if they are curved, they act like a lens. And even if the object is flat, the edges will still be visible. So the only way you'd bump into it is if it's so big that it looks like there's room between the edges. For instance, a glass door has edges that are several feet apart, so if you think that the space between them is empty, you might try to walk through that space. But if you have, say, a glass cube one foot across, that's going to be easy to see if you are paying attention and have decent lighting.