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If every object is $99$% empty space, how is reflection possible? Why doesn't light just pass through?

Also light passes as a straight line, doesn't it? The wave nature doesn't say anything about its motion. Also, does light reflect after striking an electron or atom or what?

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    $\begingroup$ Possible duplicate (even if it asks about stuff passing through each other and not invisibility specifically): physics.stackexchange.com/q/126512/50583 $\endgroup$
    – ACuriousMind
    Jun 27 at 10:16
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    $\begingroup$ Why don't bugs pass through window screens? Simplistically, of course, light waves are bigger than the holes between atoms in most solids. Atoms in gasses are further apart, so the light waves can pass through. $\endgroup$
    – jamesqf
    Jun 28 at 5:17
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    $\begingroup$ Don't discount how much of an influence that 1% can have... $\endgroup$
    – J...
    Jun 28 at 14:58
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    $\begingroup$ Pretty much the same reason that you can never actually 'touch' something. It is the interaction of electromagnetic forces that you feel, not any so called 'solid' - which don't really exist. $\endgroup$
    – Glen Yates
    Jun 28 at 15:05
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    $\begingroup$ To some extent, light does just pass through matter. That's how diagnostic x-ray imaging works, for example. However, our eyes are well adapted for detecting wavelengths that do not very much pass through most of the kinds of matter we commonly deal with. $\endgroup$ Jun 28 at 16:08

12 Answers 12

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Have you ever seen grid antennas?

enter image description here

In fact, it is also a mirror, designed to reflect the waves into its focal point.

Why it can reflect the waves if it is mostly empty space? The reason is, because the wavelength is about the same size as the holes. The wave cannot pass through the holes which are sized about the same as its wavelength.

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    $\begingroup$ An interesting analogy, but the OP could probably benefit from a little explanation about how this analogy applies to their question specifically. $\endgroup$ Jun 27 at 16:45
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    $\begingroup$ I think some explanation of "because the wavelength is about the same size as the holes" would also be useful. I don't understand it personally, because in some sense, I don't know understand what the cross sectional area of a wave is. That seems to be important, but I could be wrong (analogy: the diameter you're trying to fit through a round hole). $\endgroup$
    – Alexander
    Jun 27 at 18:55
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    $\begingroup$ @Nirvana no, it is wavelength, not amplitude which impacts this. Otherwise very bright light would be stopped by larger gaps. $\endgroup$
    – Tim
    Jun 28 at 8:00
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    $\begingroup$ @Vilx- Photons of certain frequencies can just pass between the wires. Frequencies on the visible light spectrum, for example, can pass right through them, which is why we can see gaps in the wires. This antenna was built to receive signals with a longer wavelength (and a lower frequency), such as radio waves, which can't fit through the gaps. If you looked at this antenna with a radio telescope, you wouldn't be able to "see" any gaps. $\endgroup$
    – Josh Eller
    Jun 28 at 12:02
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    $\begingroup$ @Tim sounds like Nirvana is visualising a light travelling as a sine wave-shaped beam towards the antenna. Obviously a higher amplitude makes the beam wider which stops it from fitting through gaps. I can understand the confusion caused by visualising light this way. Why is this an invalid visualisation? So many textbooks seem to enjoy drawing light like this. Why is it wavelength that matters in what sized holes light can pass through? $\endgroup$ Jun 28 at 13:57
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Yes, atoms are mostly empty, comprising a nucleus with electrons surrounding the nucleus$^1$.

But electrons are not to be considered point particles that follow fixed orbits. In fact electrons are probability clouds that are smeared around the nucleus, and fill the volume of the atom.

It is similar for bonds between atoms and in molecules, and the electron clouds fill the space in between the atoms and around them.

Also, because of the Pauli exclusion principle, no two electrons in an atom can simultaneously be in the same state, so that atoms with increasing numbers of electrons, will have these electrons with average distances further and further away from the nuclei, meaning these probability clouds must sweep an ever increasing physical volume.

Light does indeed interact with these clouds, and because of all these reasons, "seeing through" matter is not possible.

$^1$ A great majority of the matter in an atom is concentrated in the nucleus, which is very small compared to the region where the electron clouds reside. And the volume of an atom is on average about $15$ orders of magnitude larger than the volume of a nucleus. So the electrons do indeed occupy a huge portion of the volume of an atom.

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The every day world can be described with classical physics and classical electromagnetism. In this frame one can define in physics the terms "empty" and "full". A glass can be full of water , or empty of water.

The statement

every object is 99% empty space

is a misuse of of the word "empty" applying it to the microcosm of atoms and molecules. Physicists could describe this microworld only after the theory of quantum mechanics was developed and applied to microscopic observations.

Take the simple hydrogen atom , composed of an electron and a proton. The first classical mechanics image of this was the Bohr atom, and there, if you take it classically, the size of the electron and the size of the nucleus are very small, and one can classically describe "most of the hydrogen atom is empty space". This can be extended to all atoms and molecules, but experimental evidence showed that it is wrong.

The full theory of quantum mechanics developed, and there is no possibility to describe the orbit of the electron in the way one classically describes the orbit of the moon, for example, using gravitational equations. The theory predicts the probability of finding the electron in an orbital , which is the clouds referred in the other answers.

Here is where electrons can bind with a proton in hydrogen:

orbit

Any interaction with other particles for the atoms and molecules constituting matter in classical physics, at the microscopic level has to be quantum mechanical , and the "space" is not "empty" but there is a high probability that an atom or nucleus will be hit by the incoming photons, of light leading to the classical reflections etc reactions of classical matter.

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Frame challenge: Yes, each atom is mostly empty space, but there are very many of them in the path of an idealized classical light ray, and even a small amount of obstruction in each adds up.

Even if electrons were little classical spheres with a radius equal to the "classical electron radius" (which is around 5 orders of magnitude smaller than an atom), and photons were point particles moving in straight lines, a photon would have scant chance of passing through a kilometer of air without colliding with an electron. [If we assume 2 grams of matter per mole of electrons, the numbers work out to about 100 kg/m² would block half of the light rays]. So we shouldn't be able to see the sun!

This doesn't mean that the other answers are wrong when they speak about how electrons are smeared out in space. But when you run the numbers, the real mystery needing to be explained by quantum effects is how anything manages to be transparent. For this, we need to take the wave nature of photons into account -- they cannot be correctly understood as simply "particles".

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  • $\begingroup$ The problem with this model is that it only explains how light is scattered or observed, not mirror-like reflection. $\endgroup$ Jun 28 at 22:27
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    $\begingroup$ @PaŭloEbermann: The model here is not intended to explain anything; it is grossly counterfactual. I'm describing it because it appears to be more or less the model the OP was imagining in the question, only with enough extra concreteness to make some calculations. My point is that it's wrong even on that model's own premises when the OP thought it predicts that everything should be transparent to light. $\endgroup$ Jun 28 at 22:41
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In order to be absorbed or reflected a photon has to (a) pass close enough to an atom or molecule to interact with it and (b) have an energy that corresponds to a possible energy transition for the atom or molecule.

The first constraint explains why, generally speaking, liquids are more transparent than solids, and gases are even more transparent. This is because liquids are (usually) less dense than solids, and gases are less dense than liquid, so photons become less likely to pass close enough to an atom or molecule to interact.

The second constraint explains why glass is transparent to visible light, even though it is a solid. The energy gaps between electrons in glass are too wide to absorb a photon of visible light. However, ultraviolet light is energetic enough to be absorbed, so glass is opaque to ultraviolet light.

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As joseph h has said, atoms have an electron cloud surrounding them even though most of the space in an atom is empty. This electron cloud interacts, for example, with light and leads to reflection, refraction, dispersion, etc.

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It depends on what you are calling "empty".

Electrons are point particles. Or maybe they are not, but their size is probed down to 10^-22 m and found to be less than this already pretty much little value.

Atomic nuclei are not points. They are made of protons and neutrons that are not points either. But protons and neutrons are made of quarks that ARE point particles in the same sense the electron is.

That's why one can conclude that not 99%, but a honest 100% of the space is empty.

Then again, what makes the matter non-transparent (not only to light, but to other particles as well) are the interactions. The light interacts with charged particles (electrons in the first place) and this is what makes some substances opaque, reflective ot whatever.

In the quantum world, one gets used to the notion of the "interaction cross-section". This is not a real size of anything. This is a probability of an interaction expressed in terms of a virtual "area" of a target particle that will get the same probability of an interaction, if the particles were "classic" projectiles and targets.

The "classic" analogy ends when one discovers that the interaction cross-section depends in very complex manner on the energy of the particles, as well as the interaction type in the first place.

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The solar system is also mostly empty space. So why doesn't an asteroid from outside the solar system pass straight through? The solar system has many massive objects with wich the asteroid could interact gravitationally. If an asteroid would pass through the solar system it would at least be deflected by the sun a little.

Similarly light waves interact electromagnetically. The particles in atom are very small; how small depends on if you view them as point particles or probability clouds. But since they have a charge their effect can be felt at large distances. When light waves interact with the charged particles it produces disturbances in the wave pattern and when these disturbances from multiple atoms combine they can change the wave in such a way to produce effects like reflection, refraction and diffraction. For example reflection is explained in this answer

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I believe you are asking about visible light and the objects around us, so I am only going to be talking about these. To understand why everyday objects are not see through (or as you say invisible), you need to consider a few things:

First I am going to talk about solids and liquids.

  1. The everyday objects around us are made up of quantum objects, quarks and electrons, and these electrons are in a probability could around the nucleus. In fact they surround a large part of the space around the nucleus (with certain probabilities), and if you go one level higher, these atoms are in covalent bonds with each other, creating molecules, and certain electrons are surrounding whole molecules in a probability could, so when you think of objects made of these molecules, they are quite the opposite of empty space in between the atoms. The classical intuition, where you say, that photons are flying in between the nuclei fails because of this in part.

The idea that atoms are mostly "empty space" is, from a quantum viewpoint, nonsense. The volume of an atom is filled by the wavefunctions of its electrons, or, from a QFT viewpoint, there is a localized excitation of the electron field in that region of space, which are both very different from the "empty" vacuum state. When not interacting, their states are "smeared out" over the atom in something sometimes called the electron cloud, where the cloud or orbital represents the probability of finding a particle in any given spot.

Why doesn't matter pass through other matter if atoms are 99.999% empty space?

  1. Now why are these objects made up of molecules not simply see through (invisible as you say), like for example glass? It is because most objects around us, have available band gaps for electronic transitions, that correspond to visible wavelengths, and so they are able to absorb and re-emit, or reflect light in random directions, the light that is originally coming from behind the object, cannot pass through unobstructed into our eyes, giving us the illusion of see through objects, or invisibility. Only glass (and certain other materials, if you only consider solids) can do this, and refract light almost unobstructed. But most objects that you are asking about are not like this, and they absorb and re-emit or reflect the light that is coming from behind them in random directions, thus blocking the way of light from behind them, thus losing the ability to be see through (invisible). So if an object is invisible, this means that it can refract that is, let light (coming from behind it) pass through (into our eyes) it unobstructed.

  2. Then why are objects visible at all? It is because most objects around us are visible to us because they either emit light or reflect light into our eyes. The sun for example is emitting its own light, and isn't really reflecting any light, and a mirror is reflecting most light and is not emitting much of its own (in the visible range).

Now on to gases. Why is the atmosphere see through? Light does scatter (Rayleigh scattering, makes the sky blue) off the atoms in it. Then why can we see through it? It is because on the one hand, atoms are really not densely packed, and they do not bond in any way, so there is in this case the space (that you mention) for light to pass in between the atoms unobstructed into our eyes, and, on the other hand, even if some light interacts with the atoms, most of the visible light scatters off the atoms elastically, without absorption, only changing direction.

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As other posters pointed out, the notion that space is "empty" is problematic for other reasons, but you don't have to go into that detail to understand the answer to your question. It's much simpler:

The reason is the same why you can sit on your chair instead of dropping through the "99% empty" chair to the floor.

The particles that make up the chair, and your body (and the floor, and the bedrock your house is built on, etc.) all exert forces that work at quite a distance from the particles themselves. According to the standard theory, there are four such forces, known as gravity, electromagnetic, strong force and weak force (you don't have to worry about those details to get your question answered. Just know that all of them work a certain, small or large, distance away from the particles themselves).

Basically, the "empty" space is actually full of those forces. Those same forces also act on the photons that make up light. The photons thus bounce around not off particles themselves, but essentially off the forces. As an aside, the assumption that light travels in straight lines through space is also false, because light can, and does, get deflected by gravity.

Your question is actually much more insightful than it seems at first glance. There really is (probably) a kind of matter that acts the way you thought: dark matter. This is because, according to the most prevalent theories, dark matter particles do not exert most of the forces (only gravity). Light simply passes through clouds of dark matter. Caveat: this is currently subject of intense research, not yet firmly established as certain fact.

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As others have said, electrons aren't really point-particles in an atom, but rather smeared orbitals that take up some volume of space (etc.).

It may be of interest to consider this record-setting image released last week of atomic structure of praseodymium orthoscandate, zoomed in 100 million times, taken by researchers at Cornell via electron ptychography. Based on this, you can see the space effectively taken up by the atoms, and you can also note the deeper atoms which, taken together, are going to block visible light (and this experiment only used material a few dozen atoms thick).

Image shows an electron ptychographic reconstruction of a praseodymium orthoscandate (PrScO3) crystal, zoomed in 100 million times. Credit: Cornell University

From the Scientific American article linked above:

The researchers obtained the image using a technique called electron ptychography. It involves shooting a beam of electrons, about a billion of them per second, at a target material. The beam moves infinitesimally as the electrons are fired, so they hit the sample from slightly different angles each time—sometimes they pass through cleanly, and other times they hit atoms and bounce around inside the sample on their way out. Cornell physicist David Muller, whose team conducted the recent study, likens the technique to playing dodgeball against opponents who are standing in the dark. The dodgeballs are electrons, and the targets are individual atoms. Though Muller cannot see the targets, he can see where the “dodgeballs” end up, thanks to advanced detectors. Based on the speckle pattern generated by billions of electrons, machine-learning algorithms can calculate where the atoms were in the sample and what their shapes might be.

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yes , in a way it is a good question because in junior class also we have learn about atoms and we know that almost all masses of atoms are concentrated at its center in atomic nucleus , but the radius of atomic nucleus is 1/100,000 of the total radius of the atom. So this means the radius of atomic nucleus is very very small comparative to radius of whole atom.

If observe closely this statement you will understand one thing that electrons which revolve around nucleus do also interact with sorrounding because it has charge , this means masses are although concentrated at centre but the area for the interaction have been increased. Means for example if we take a Box , in context with mass it is largely empty but it look for Interacting charges from that box , it will sorround all box , in each and every place because charges interact everywhere. Now this means that although A Matter has mass concentrated in very less area but the charges are spread everwhere.

Now the question is why do we see everthing, Lights are composed of Photos , something which does not have any mass but do have interacting charges because it is Electromagnetic wave, Now this means it has some charge and this charge is one which interacts with any glass which also has charges and also have potential to develop magneic field . Now as there electromagnetic waves , interact with the electron present in the glass the elctrons present in the glass gets the identity of that waves and starts behaving as electromagnetic waves .

see this not makes everting clear, but give one more shot , see waves are nothing but the charges which are oscilating , so this means light does not have any identity only it has a constant momentum which is carried from one place to other . So when it interact with the charges In the glass it transfers the momentum as per (the conservation of momentum )and this momentum converts this free elctron in the glass to oscillate at the same rate as of the light . Now this waves created are spread in all direction but the prominent waves follow the same angle as of the angle of incidence the light. In this way these waves reaches our eye and we are able to see things. This means empty spaces are not really empty but full with possiblities which we are yet to know.

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    $\begingroup$ Unfortunately this does not answer the question $\endgroup$ Jun 29 at 16:50

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