atoms and molecules

[Cristatus]

my physics teacher asked me this question two years ago, and then i asked him the same question last year, and he said he didn't know the answer.

anyways, the question is:

if atoms and molecules are so tiny that they can not be seen, how can we have objects that are solid, and can still be seen through? is this one of nature's paradoxes?

 

Well, the majority of the space occupied by an atom is, well, empty space. The only thing that makes objects solid is the repulsive effects between electrons of different objects, say your hand and a wall. As for being able to see through a solid object, I imagine the "see through" part has to do with the structure of the atoms in the material in question as well as the density of said atoms.

 

[Cogman]

just like stone monkey said. I remember a example to how tiny and yet how much empty space there is in an atom. Imagine a Giant football arena. Now imagine a paper clip. If the paperclip represents the nucleous of the atom, and is placed in the center of the feild. The electrons would be on the very top bleachers and farther out.

 

[sao123]

It has everything to do with the way the atoms and molecules are positioned.

Most of the solid objects which are transparent have a crystal structure.
Crystals have their molecules aligned in layerers. How many bonds exist between the layers determine the strength of the crystal. This is the defining difference between say graphitee and diamond. both of which are crystals made of carbon. Graphite carbons are layered but organized tightly which makes it block light from passing through it. Diamond on the other hand has highly organized spread carbon atoms which allow light to pass between the atoms the entire way through the whole diamond. (Imagine 2 isles at the supermarket, one cluttered, and one clear, you as a lgith ray can pass through one, and not the other.) Also noteworthy is Diamond has the most interlayer bonds, which makes it the strong crystal. Coal layers have fewer interlayer bonds which is what makes graphit brittle.

Although this description is crude and does not reflect every principal involved, it should give you a starter idea. Try researching crystal lattice structure... and here are some diagrams.

 

[silverpig]

It mostly has to do with the energy structures of the electrons. The thing that blocks light in an opaque solid is absorption/scattering off of the electrons. If the atoms are arranged in such a way that there are no electrons that "want" to absorb light in the visible range of the spectrum, then they will be transparent.

Look at some plastics. They are transparent to us in the visual, but are almost completely opaque in the UV (UV sunglasses anyone?). This is because the atoms absorb light in the UV range very well, while they don't have any electrons in an energy level which would absorb a visual photon.

 

[DrPizza]

Originally posted by: silverpig
It mostly has to do with the energy structures of the electrons. The thing that blocks light in an opaque solid is absorption/scattering off of the electrons. If the atoms are arranged in such a way that there are no electrons that "want" to absorb light in the visible range of the spectrum, then they will be transparent.

Look at some plastics. They are transparent to us in the visual, but are almost completely opaque in the UV (UV sunglasses anyone?). This is because the atoms absorb light in the UV range very well, while they don't have any electrons in an energy level which would absorb a visual photon.

^^probably the best explanation.

I don't like the crystal explanation with the open space because it doesn't account for one phenomena: refraction of light as it passes through. If we were to claim that the light was passing through the "empty space" between crystals, and the empty space between the electrons and nuclei, how could we explain how the speed of light is lower in the solid?

 

[GimpyOne]

The crystal structure idea is actually bad explaination as well because it completely neglects the most common tranparent object in the world. Glass. Not to mention the fact that the majority of polymers that are tranparent are amorphous not crystalline.

Oh well, anyway, the energy level diagram explaination is the correct one. Phonon/Photon absorbtions control it all.

 

[silverpig]

This is actually how they "slow" or "stop" light in a medium. They very very carefully arrange the electrons in sodium atoms to have a very specific configuration so there's a very narrow band pass for light to get through. Basically it will allow light at say, 523 nm to pass through, but not 524-560 or 480-522 nm (pulling those numbers out of my ass, but the idea is there). They then shine light in a range from say 490 nm to 515 nm at the gas and this light is basically squeezed through the 1 nm band pass gap. The phase velocity of light in the medium has a relation to the bandwidth and the group velocity (c) such that a very narrow band pass makes the phase velocity a substantially small fraction of c, allowing you to slow the apparent progression of light to say 50 km/h through the sodium gas.

This is very crude, but the basics are there.

 

[Cattlegod]

well, the resolution of your eyes is terrible in relation to the size of an atom, also the speed of the particles in an atom are way to fast for our brain to process - hence, it comes out as a blur.


that is like saying why can't we accelerate with 10000g's of force? we physically can't do it.

 

[gururu]

transparent solids let light through...unmodified light, so the image from the other side is not corrupted enough to distort our perception of the image. warped plastic and glass will bend light, warping the image but still transmitting most of the visible image. the only way photons can pass through a solid is if their wavelength is sufficient to pass through the matrix/lattice with minimal perturbation. the threshold is between visible and ultra violet in most cases as glass for example allows radio, infrared and visible to pass through. the reason ultra violet is effectively blocked is because it completes more cycles in a shorter distance, making more contact with atoms in the matrix, which inevitably end up disrupting the original path of the photons.

 

[sao123]

The crystal structure idea is actually bad explaination as well because it completely neglects the most common tranparent object in the world. Glass

Indeed it does not. Glass is a common form of a silicon dioxide crystal, which happens to fall within the same family as most naturally occuring quarts crystals.

 

[silverpig]

Originally posted by: sao123

The crystal structure idea is actually bad explaination as well because it completely neglects the most common tranparent object in the world. Glass

Indeed it does not. Glass is a common form of a silicon dioxide crystal, which happens to fall within the same family as most naturally occuring quarts crystals.

How about air then? Water?

 

[sao123]

It mostly has to do with the energy structures of the electrons. The thing that blocks light in an opaque solid is absorption/scattering off of the electrons. If the atoms are arranged in such a way that there are no electrons that "want" to absorb light in the visible range of the spectrum, then they will be transparent.

Look at some plastics. They are transparent to us in the visual, but are almost completely opaque in the UV (UV sunglasses anyone?). This is because the atoms absorb light in the UV range very well, while they don't have any electrons in an energy level which would absorb a visual photon.

None of what i said disagrees with this. Absorbsion and reflection must occur to see an an oppaque object. But the light must pass through un absorbed or reflected for an object to be transparent or translucent.

I don't like the crystal explanation with the open space because it doesn't account for one phenomena: refraction of light as it passes through. If we were to claim that the light was passing through the "empty space" between crystals, and the empty space between the electrons and nuclei, how could we explain how the speed of light is lower in the solid?

I dont believe anyone has said or proved that refraction cannot take place on an atomic/molecular scale. Actually scientists are doing exactly such experiments now which predict refraction can take place in the presence of only 10 atoms.

How about air then? Water?

as you are well aware neither air or water a solid as the first question asked.
However the very same principle applies. The very state of the matter solid, liquid, gas, is determined by the amount of thermal energy in that particular piece of matter. The more thermal energy a group of molecules has, the more kinetic energy that group of molecules have. As molecules gain more kinetic energy, they spread out more. (As an objects is heated, it expands, i believe you know that.) The space between the molecules increases. With an increase in space between molecules, organization and alignment are not as crucial to allowing light to pass through as they are with a solid structure.
Thus the increase in space allows more light to pass through between the molecules, which is why most liquids and gases are translucent.

 

[silverpig]

Originally posted by: sao123

It mostly has to do with the energy structures of the electrons. The thing that blocks light in an opaque solid is absorption/scattering off of the electrons. If the atoms are arranged in such a way that there are no electrons that "want" to absorb light in the visible range of the spectrum, then they will be transparent.

Look at some plastics. They are transparent to us in the visual, but are almost completely opaque in the UV (UV sunglasses anyone?). This is because the atoms absorb light in the UV range very well, while they don't have any electrons in an energy level which would absorb a visual photon.

None of what i said disagrees with this. Absorbsion and reflection must occur to see an an oppaque object. But the light must pass through un absorbed or reflected for an object to be transparent or translucent.

I don't like the crystal explanation with the open space because it doesn't account for one phenomena: refraction of light as it passes through. If we were to claim that the light was passing through the "empty space" between crystals, and the empty space between the electrons and nuclei, how could we explain how the speed of light is lower in the solid?

I dont believe anyone has said or proved that refraction cannot take place on an atomic/molecular scale. Actually scientists are doing exactly such experiments now which predict refraction can take place in the presence of only 10 atoms.

Thompson scattering? Compton effect? You can have light bending about single electrons.

How about air then? Water?

as you are well aware neither air or water a solid as the first question asked.
However the very same principle applies. The very state of the matter solid, liquid, gas, is determined by the amount of thermal energy in that particular piece of matter. The more thermal energy a group of molecules has, the more kinetic energy that group of molecules have. As molecules gain more kinetic energy, they spread out more. (As an objects is heated, it expands, i believe you know that.) The space between the molecules increases. With an increase in space between molecules, organization and alignment are not as crucial to allowing light to pass through as they are with a solid structure.
Thus the increase in space allows more light to pass through between the molecules, which is why most liquids and gases are translucent.

Water is denser than ice. The molecules are closer together. Yet water is still transparent.

Think about it. Imagine a simple crystalline structure that is cubic in nature. The atoms are arranged in layers of squares. Now make a spherical ball of this material. Look at it along one of the axes along which the atoms are aligned. According to your crystalline structure argument you should see right through the object because you can look down the "halls" between the atoms. Fine. Now rotate your sphere slightly in two orthogonal directions (right and up say). Now you should be looking down the halls on an angle. No light should get through. But if you have a ball of transparent material, it's transparent no matter which direction you look at it...

 

[silverpig]

Due to electron energies

The reason why UV light is blocked more easily many materials which are transparent in the visual is because the UV radiation carries more energy. In these materials the electrons are in such a state where the next available energy level is too far away from the currently occupied highest energy level for a visual photon to boost the electron up to the next level. Thus the visual photons will go by unabsorbed. The UV photons in the case of this materials do have enough energy to boost the electron up a level, so they are absorbed by that electron, then shot back out with the electron relaxes back to its preferred lower energy state.

 

[MobiusPizza]

The definition of "seen" is important.

Hell I can beat your physics teacher. I'm just 18 here lol
The fact that we are able to see is that matter deflect light at certain wavelength and angles. Single atom or molecule cannot deflect enough light/can't deflect at all to allow us to even see. However when they join up to form a structure, more light deflected by a certain range of wavelengths (colour) and angles (shiny/dull) allow us to "see" the object.

 

[Mday]

| | | | | | |

if you look up and down, you can see through that. if you look at an angle, what you can see depends on the angle... fairly large angles are viewable

/\ /\ /\ /\ /\ /\ /\ /\ /
/\ /\ /\ /\ /\ /\ /\ /\ /

if you look up and down, you can barely see through that. if you look at an angle, what you can see depends on the angle... fairly small angles

XXXXXXXXXXXXXX

if you look up and down, you can see through that even less. if you look at an angle, what you can see depends on the angle, VERY small angle

////////////////////////////

if you look up and down, you cant through that. if you look at an angle, you see even less, depending on the angle you can see through it