Question

what is the criteria for a material to transmit light through it i.e why are substances colourless ,translucent or opaque?

Answer 1

This one is for why is glass transparent:

Photons pass through glass because they are not absorbed. And they are not absorbed because there is nothing which "absorbs" light in visual frequencies in glass. You may have heard that ultraviolet photons are absorbed by glass, so glass is not transparent for them. Exactly the same happens with X-rays for which our body is nearly transparent whilst a metal plate absorbs it. This is experimental evidence.

Any photon has certain frequency - which for visible light is related to the colour of light, whilst for lower or upper frequencies in the electromagnetic spectrum it is simply a measure of the energy transported by photon. A material's absorption spectrum (which frequencies are absorbed and how much so) depends on the structure of the material at atomic scale. Absorption may be from atoms which absorb photons (remember - electrons go to upper energetic states by absorbing photons), from molecules, or from lattices. There are important differences in these absorption possibilities:

1.Atoms absorb well-defined discrete frequencies. Usually single atoms absorb only a few frequencies - it depends on the energetic spectrum of its electrons. Regarding atomic absorption, the graph of absorption (plotted as a function of frequency of light) contains well-defined peaks for frequencies when absorption occurs, and no absorption at all between them.

2.Molecules absorb discrete frequencies but there are many more absorption lines because even a simple molecule has many more energetic levels than any atom. So molecules absorb much more light.

3.Crystalline lattices may absorb not only discrete frequencies but also continuous bands of frequencies, mainly because of discrepancies in the crystalline structure.
As glass is a non-crystalline, overcooled fluid, consisting of molecules, its absorption occurs in the 1st and 2nd ways, but because of the matter it is composed of, it absorbs outside our visible spectrum.

Answer 2

@quarkine This one is for why is the velocity of light in glass less. I remember you asking this question long ago and since today i found this one, I am going to share it with you:

Glass does absorb photons - they are absorbed by the inter atomic bonds (phonons) and re-emitted, this is essentialy why the speed of light in glass is slower. It appears transparent because the direction of the light is preserved by the ordered bonds and because little of the energy is lost

Source: not my brain.. this one: http://physics.stackexchange.com/questions/7437/why-glass-is-transparent

Answer 3

check this out too..
http://www.youtube.com/watch?v=Omr0JNyDBI0

Answer 4

And interesting fact: some (skeptic) people say "do you see electrons?" what would the answer be??

Answer 5

That video is awesome and explains a lot.. Worth watch!
Except for the last sentence, which has a slight error. But then there is a message regarding the last sentence right below the video.

Answer 6

@ujjwal thnx buddy.guess too materials!! will take a while to digest all of it.
well, in video he says about the energy level gap of electrons being too high.but wont that according to the band theory imply that every transparent substance is an insulator and vice versa?
@experimentX : no ,i hope!!

Answer 7

yes ... all objects that we can see it 99.99% electrons

Answer 8

@quarkine it would imply that every transparent object is insulator.(probably).. Not very sure about the vice-versa condition.

Answer 9

@experimentX but 'seeing' is basically light falling on something and reflecting back to our eyes.so if an electron were to be in way of light photons so wont they be completely absorbed or just pass and no reflection will take place??
@ujjwal yeah i just reached about 75% of vid and saw that he is actually explaining it in terms of band theory...slow connection :(

Answer 10

@quarkine Oh! so the new question is why does reflection take place?

Answer 11

electrons in atom are in quantized state ... according to Bhor's model ... electrons also have frequency and wavelength ( from deBroglie wave matter ) (the integer multiple of wavelength must be equal the orbital circumference)

most likely, if the frequency (energy) of light must be matched so that i can excite electron to it's another quantized orbit.

Also electrons are not just discrete harmonic oscillators, rather they form a band ... and absorb particular band of EM waves.

Answer 12

@experimentX what has the debroglie wavelength of electron got to do here?

Answer 13

hmm ... that the reason for stability of atom ... explains Bhor's model

Answer 14

@ujjwal haha..we both asking simple but crazy questions!!
@experimentX :me too not sure if de broglie wavelength has to do with absorbtion of light..

Answer 15

In case of particular say (hydrogen), i think if the energy of light is enough to excite electron to another state ,,, the light will be absorbed. If it is not enough then it will simply be reflected or transmitted ... this phenomenon is highly probabilistic.

Answer 16

Hmm ... it's nothing to do with absorption,
but it helps to understand Bhor's model
2pi r = h/mv

Answer 17

If it is not transmitted, It is either reflected or absorbed. what determines whether it is reflected or absorbed?

Answer 18

not quite sure ... may be this
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/barr.html

Answer 19

also this could happen http://en.wikipedia.org/wiki/Compton_scattering

Answer 20

for metal surfaces,
When visible or ultraviolet (UV) radiation shines on a metal, the photons are absorbed by the electrons in the metal. The electrons are then excited up to a higher energy level. When an electron returns to a lower energy level, another photon is emitted. This is how light appears to be reflected off a metal surface.

Answer 21

half of my understanding of waves vanishes when it is said that EM waves can travell without any media.the whole concept of wave as a disturbance in a medium is not followed here.so i've no idea how they are absorbed or transmitted..
@physicsme : but then how do they follow law of reflection??

Answer 22

@experimentX : ur link is kinda scary :) not elementary at all!!

Answer 23

I'm not pretty sure myself ... we will talk in detail after i do detail study after i finish my exams

Answer 24

@quarkine In case of metals, i don't think they precisely follow law of reflection. light is scattered off randomly.
@experimentX LOL.. I read that article regarding Compton Scattering earlier today. (what a co-incidence).
So, basically, the point is that if the energy of photon is greater than energy gap, a new kind of photon (with different wavelength) is reflected back, which accounts for the color of objects we actually see.

But then i have one question: In case of black bodies, the energy gap is such that it absorbs all wavelengths? How is it possible? How can all wavelength be absorbed? Take lamp-black as an example, not black hole.. :p

Answer 25

@quarkine @ujjwal the laws of reflection show the wave nature of light, the rays reflecting off. In many books, we see that they do not really omit metals from the laws of reflection.
And absorption of photons, is the particle nature so maybe we'll need to get more into the wave-particle duality to understand that....

Answer 26

@experimentX : oh ur exam going on.. sry to take ur time..the link says according to 'classical phy'..so are the tunneling,barrier penetration etc explainable by classical phy?
@ujjwal : if law of reflection were not to work for metal surfaces then it shouldn't for mirror too as mirror itself a silver painted surface..

Answer 27

if i remember that ... i read quite a bit of it in waves chapter ... and a lot more in QM. tunneling is certainly and Quantum Phenomenon

Answer 28

@quarkine I mean to say, the photon is first absorbed by metal and than given off (when it fall back to lower state). And how would the metal know in which direction it is to direct the emitted photon?

Answer 29

exactly.thats why i'm not intuitively convinced about this mechanism..i feel like there may be some thing more to it..
though the direction may be dictated by conservation of momentum ,in some way..

Answer 30

No, i don't think its compton scattering in case of metals. the wavelength remains unchanged. Not 100% sure though.

Answer 31

we would take conservation of momentum into account if the photon wold be simply bouncing off the metal surface. But here the photon is first absorbed and then emitted. It is not bouncing off.

Answer 32

thats why i said 'in some way' at last...coz i dont know :)

Answer 33

lets hope, we get the answer soon.

Answer 34

yeah lets try it after research..at least we know why light travell through glass...next time ,and i quote,
"so the new question is why does reflection take place?"

Answer 35

LOL.. :D

Answer 36

BTW i meant 'sm research' there nt the Phd research..hahaha...
thanx everyone...

Answer 37

welcome buddy!

Answer 38

well ... it was a nice discussion .. :)