How do some things (air, water, etc) let light pass through them, and not affect them? I can understand how some things absorb all the spectrum colors (black), and some reflect them all (white), but how does it prevent light from affecting it, or prevent it from affecting it very much? Thanks for you help!

By default any object will be transparent to a certain wavelength (color) of light, unless that wavelength is of the exact energy needed to cause something to happen in the object, like promoting an electron to a higher energy level. The molecules in air aren't effected much by the visible wavelengths of light, so most of the time light will pass through the air without having an effect on it.

Blue ligt starts to come close to a resonant frequency in some component of air, that is why the sky scatters blue light down and sunsets are red (red from the loss of blue light).

Not quite; the particles in the upper atmosphere are far enough apart so that Rayleigh scattering occurs, which favours blue light as you say, but does not scatter red light anything like as much.

http://www.sundog.clara.co.uk/atoptics/sunsets.htm#ray

As you can see from this link there are other types of scattering as well, but the lower atmosphere is largely free of scattering effects, and therefore more or less transparent when dry.
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could it be because it's clear? :D

Or maybe it's that gas has less molecules that the photon could interact with :bugeye:

a molecule has a certain distance across and a certain density.
the energy that is traversing this distance must be at the
same frequency or the power of the energy must be great
enough to cause the molecule to vibrate, If vibration occurs
the energy will be lost and a reduction in the light coming from the molecule will appear deminished.

strategicman,

great question! i got answers to something i THOUGHT i already knew, thanks.

when i first read it i kind of went, "well, duuuh! :bugeye: who doesn't know that?" - sorry i tend to have a knee jerk 'everyone but me is a idiot' complex before i think about it for a minute.

interesting stuff!


buff

Thanks Buff. Yeah, I'm kinda like that too :D

if i understand the responses, (putting it very basically) everything is transparent and everything is opaque depending on what wavelength of light one sees it thru (eg. x-rays show our flesh and muscle as transparent and in infrared steam is opaque), is that roughly correct?


buff

Originally posted by Specialist
Or maybe it's that gas has less molecules that the photon could interact with :bugeye:

^^^^AFAIK, that's it^^^^

if you were to take an air molecule at one atmospheric pressure, and increased it's atoms diameter to that of an orange, and placed this orange-sized-atom molecule in LA, the next closest orange-sized-atom molecule would be somewhere near NewYork City. lots of room for photons to pass through un-interupted.

Consider mirages. where heat has spead up the air molecules enough that light entering the area is almost all bent, allowing for an object miles away to appear mere yards away. by speading up the molecules, you have caused a higher rate of interaction between air and light, resulting in air that is not really "clear".

Air is "colorless" because the majority of gas which makes up air is colorless. colorless!=clear, keep in mind.


edit: the colorless thing applies to what people have said above- you need certain frequencies of light to be absorbed or reflected/refracted.

Originally posted by buffys
if i understand the responses, (putting it very basically) everything is transparent and everything is opaque depending on what wavelength of light one sees it thru (eg. x-rays show our flesh and muscle as transparent and in infrared steam is opaque), is that roughly correct? This is true. Water and glass are transparent to visible light, but block infra-red light. Most of the human body blocks visible light but is transparent to x-rays. It all depends on what wavelength of light you're looking at.

most liquids and gases appear transparent because the atoms and molecules of which they are constructed are aligned randomly so that there is plenty of room for photons to fit through, however here and there a photon will eb absorbed and throufg large amounts of any substance light cannot be seen, note; being in deep water for example. however there are some solids which also permit light to travel through them, at this point there is a sub molecular proces accouring. When a photon (of any region of the spectrum) hits an atom a number of things can ocour
1) the atom can absorb teh energy and an electrom will move to a higher state within it, this can either be stored or released in another frequency e.g. metals often release heat, and phosphorus turns UV into visible light
2) the electron can jump revolve once and release the photon back in the opposite direction note: reflection
3) the atom may not be able to absorb the energy and the electron may continue uninhibited

the latter occors with glass and some forms of plastic, note that all the substances that this occurs with have been melted and cooled thus their atoms are also randomly aligned

Our eyes happen to be sensitive to a set of frequencies of light that traverses through air molecules without being affected. That's why we can see. There's a much bigger spectrum of electromagetic frequencies that we can't see, and also a pretty big spectrum that doesn't pass through the atmosphere easily. (That's why AM radio bounces off the clouds and can be picked up a thousand miles away, while FM just shoots off to Tau Ceti.)

If an animal had evolved with eyes that were sensitive to a much lower frequency of radiation, he wouldn't be able to see very well because the air would play tricks on him. Like the mirages in the desert except all the time. That species wouldn't survive for very many generations. The one that survived to become the ancestor of all the higher phyla of animals with eyes was the one that could see what we call visible light.

It had to be that way. Nothing else would work, Grasshopper.

Originally posted by god-of-course
most liquids and gases appear transparent because the atoms and molecules of which they are constructed are aligned randomly so that there is plenty of room for photons to fit through, however here and there a photon will eb absorbed and throufg large amounts of any substance light cannot be seen, note; being in deep water for example. however there are some solids which also permit light to travel through them, at this point there is a sub molecular proces accouring. This isn't really correct. Most gases and liquids are transparent because their molecules don't absorb visible light. It doesn't have much to do with how the molecules are ordered, i.e. whether they are arranged randomly or in a lattice.

Occasionally the molecules of a substance will form complexes that will themselves absorb light, as in the case of water. H2O molecules arrange themselves into lattices because of hydrogen bonding, and the bonds between the molecules in the lattice tend to absorb light in the red part of the spectrum, making the light that comes out look blue. This is pretty unusual, though. Most of the time the relative order or disorder of the molecules doesn't matter.When a photon (of any region of the spectrum) hits an atom a number of things can ocour
1) the atom can absorb teh energy and an electrom will move to a higher state within it, this can either be stored or released in another frequency e.g. metals often release heat, and phosphorus turns UV into visible light
2) the electron can jump revolve once and release the photon back in the opposite direction note: reflection
3) the atom may not be able to absorb the energy and the electron may continue uninhibitedThere are also all sorts of scattering processes that can occur. In scattering the photon is not actually absorbed, but 'bounces' off in a different direction, sometimes at a different wavelength. Scattering is what causes the sky to be blue, since nitrogen scatters blue light more than other colors.the latter occors with glass and some forms of plastic, note that all the substances that this occurs with have been melted and cooled thus their atoms are also randomly alignedConsider a perfect quartz crystal; it's completely transparent even though the molecules in it are highly ordered.

This thread makes me wonder about the science of invisibility!
I just saw Hollow Man recently there on TV and Kevin Bacon was trying to change the molecular structure of organic compounds (I presume) so that they will transmit light and also be stable at the same time. In the movie of course, the biggest problem was keeping the molecules stable. Does anyone think that this sort of thing could ever happen? That normally opaque solids (be they organic or not) can be made to transmit light???