Naked Science Forum
General Discussion & Feedback => Radio Show & Podcast Feedback => Topic started by: thedoc on 11/05/2010 17:21:07
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Is there a cure for spots? Why do we cry? Does alcohol really kill brain cells? It's a Question and Answer Extravaganza on this week's Naked Scientists! We find out what makes a Chameleon change colour, why birds fly into windows and how a hair can change colour along it's length. Also, witnessing the birth of stars, the Neanderthal genome and how washing your hands can change the way you think. Plus, Meera dabbles with green gadgets and smell-free toilets in the home of the future, and Dave shows you how to build a hovercraft in Kitchen Science.
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How can we see through such an amorphous material as glass. If it were a crystalline solid, I could see the solution, but a randomly structured material like glass? Surely it should be like looking through cheap plastic windows. But it isn't. Why not?
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Hi John
I'm not sure I can understand the problem - can you explain in slightly more detail?
Chris
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Hi Chris,
Why don't the photons get scattered randomly as with an opaque material, such as plastic sheeting, resulting in not being able to see an image on the other side? Glass is an amorphic solid isn't it? The textbook diagrams liken refraction to rows of photons all slowing down in the material and speeding up again when exiting the material. But why don't the rows get scattered when encountering the material. Generally, what is the feature that distinguishes transparent material from opaque material? (Have I explained that clearly enough?)
John
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Hi John
Thanks for the clarification.
One way to look at this problem is to compare liquid water and snow. Both are molecularly identical and composed of H2O, but one is opaque - and highly reflective - the other transparent. You could also consider a bucket of water into which you tip a capful of Dettol, making the water go white.
The reason for these opaque, reflective effects (snow and dettol) is down to the particle size. In snow, when light passes from the air into the ice crystals it slows down and refracts (bends). Leaving the ice again the light re-enters air and bends / refracts again. Done enough times - because there are many particles surrounded by air - this has the effect of returning to your eye all the wavelengths of light so you see a brilliant white surface.
But liquid water does not behave in this way because it is not individual particles surrounded by air; the optical density / refractive index of the medium is therefore relatively homogeneous, so the light is not refracted multiple times and so you can see through it.
And the dettol in water?
I gave this example to prove my point because dettol in water produces billions of tiny micelles - oily bags. So light passing into the water is continuously passing from water into oil and back into water, refracting with every change of medium. This makes the liquid like the snow crystals and it returns all wavelengths to your eye, making the water look white.
So, to summarise, a white, opaque surface - like titanium-based paint - refracts the light hitting it to the extent that all wavelengths are returned to your eye, making the surface look white. A transparent substance does not cause multiple refractions like this and so you can see what is on the other side of it. But because it does refract the light a bit - as it goes in and out - light passing through is bent and so your eye can perceive where the transparent material is; otherwise it would look invisible. Dave has an excellent demonstration of this as a kitchen science in which he immerses a pyrex bowl in cooking oil. As the refractive index of oil and pyrex is nearly identical, the immersed bowl vanishes!
Chris
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Hi Chris,
Thank you, that's a very good explanation for the effects at the microscopic level, but I'm really trying to understand it at the level of a single photon. What exactly happens when an individual photon encounters materials such as diamond and glass. With diamond I could see that the regular lattice structure allows the photon to, as it were, pass down the tunnels in the lattice without interacting much with the individual atoms (does it?). But with an amorphic solid such as glass, surely it would be randomly scattered by an atomic nucleus within a few nanometers. It isn't, but why not?
Or does it have anything to do with the inter-atomic separations and the probability of an individual photon being scattered by a nucleus? But that surely can't be the explanation either, as a photon isn't a neutrino.
A puzzled John...
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Hi John
You have to remember that photons can behave both as discrete particles (as in the photoelectric effect) and also as waves (as in diffraction); it's probably therefore unhelpful to try to consider particles fitting through holes in a substance.
The bottom line is that light is an electromagnetic wave; when it interacts with a substance the electrons in that substance soak up the energy in the light wave and then re-release the energy, re-generating the light wave. This happens sequentially as the "light" passes through the material, with electrons handing on the energy from one to the next. At a simple level, this process takes time, which is why light travels more slowly in a more optically-dense (higher refractive index) substance.
Chris
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Hi Chris,
OK I'm thinking about cats... Your explanation makes sense but applies to any material, whether opaque or transparent. Is the difference between them the energy required to raise the energy level of an electron in the outer ring... transparent = little, opaque = a lot?
If I assume that in glass a (visible wavelength) photon contains just the right amount of energy to raise an electron to a higher energy level and that within a short space of time it decays again re-emitting a photon, then why do the photons travel in a straight line in glass? Is it due to the conservation of momentum? Or is there another explanation?
And what (once again) is the difference between opaque° and transparent substances that dissipates the photons in opaque substances, but allows them to travel in straight lines in transparent ones? The conservation of momentum solution applies, after all, to all substances, so that seems to rule it out, at least as the complete solution.
John
PS° For 'opaque' use e.g. plastic sheeting, as that allows the photons to pass through, but not in straight lines.
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Hi John
Remember that "transparent" and "opaque" apply to a substance only in terms of the wavelength of the light you are dealing with.
For instance, microwaves and visible light are both "light", but the grille on the door of a microwave oven is opaque to microwaves - to protect the user - but transparent to visible light, which can still exit allowing you to see the food cooking inside.
The reason is that the wavelength of visible light is a fraction of the wavelength of the microwaves, which are impeded by the metallic grille. The visible light, however, slips through the gaps in the metallic mask quite easily.
Conversely, mobile phone signals travel quite nicely through the walls of houses, but visible light doesn't.
As a visual demonstration of this, for Kitchen Science, Dave has turned a webcam into an infrared camera; pointed at an apparently opaque, black bottle of Coke, this IR camera sees straight through as though the Coke is colourless.
Chris
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Hi Chris,
That's it! Many thanks for taking so much trouble about this.
John