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Author Topic: Cling film and colours  (Read 11021 times)

Offline anna

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Cling film and colours
« on: 09/04/2003 14:40:46 »
i am studying teaching and as part of the course i do science. i have to give a 15 min presentation as to why, when you place cellophane over pastel coloured paper, it changes colour and why it changes to the colour it changes to. i know all i can find out about absorption, reflection, the primary colours, cones, rods and whatever else but im still confused! i am not the most science minded person so please be patient.....thankyou...


 

Offline george

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Re: Cling film and colours
« Reply #1 on: 09/04/2003 17:04:04 »
Hi Anna,

Funny how you never notice things until someone points them out, then you can't help but notice it.

I'm wondering if the observed effect is true only of cellophane or whether the same occurs with a pane of glass ?

I suspect that it will only happen with cellophane and is related to the composition of the cellophane causing it to absorb light of certain wavelengths whilst allowing other wavelengths to pass through unimpeded, thus distorting the spectrum of light coming from a coloured piece of paper and hence apparently changing the colour of the paper.

On a gross scale I suppose it's a bit like wearing red tinted sunglasses - when viewing red objects through these glasses they appear black - because all of the red light reflected by a 'red' object is also refelected by the glasses (which is why they appear red) and hence no light goes through and your eye sees a black version of the 'red' object ? Clear as mud !

Any physicists around here lend some muscle to this problem ?

Great question though, can't wait to hear what other people reckon,

good luck, George
 

Offline chris

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Re: Cling film and colours
« Reply #2 on: 10/04/2003 14:06:43 »
I don't agree with this. If what you say is true about the cellophane absorbing light of certain wavelengths then the colour should remain the same when viewed through the cellophane whether it is against the paper, or set apart from the paper.

We think it's down to refraction and to do with the thickness of the cellophane.
 

Offline Donnah

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Re: Cling film and colours
« Reply #3 on: 25/04/2003 20:05:09 »
Over what period of time does this color change take place?  I put cellophane over pastel paper and didn't see any significant difference.
 

Offline Quantumcat

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Re: Cling film and colours
« Reply #4 on: 03/05/2003 18:23:36 »
"If what you say is true about the cellophane absorbing light of certain wavelengths then the colour should remain the same when viewed through the cellophane whether it is against the paper, or set apart from the paper."

Ever think about he fact that there is other light apart from that which is going directly from the paper through the cellophane into your eye? :p

When the cellophane is futher away, stray light can come in between the paper and the cellophane and make the colour lighter/brighter. Moving the cellophane closer prevents stray light from getting in between, and so the colour becomes darker.
 

Offline chris

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Re: Cling film and colours
« Reply #5 on: 04/05/2003 11:00:45 »
But what about the gap between the cellophane and your eye ?!
 

Offline Quantumcat

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Re: Cling film and colours
« Reply #6 on: 05/05/2003 04:54:59 »
Since everything isn't completely and utterly controlled, the light isn't going in perfect parallel straight lines, so the distance between the light beams, if you can say that, becomes more and more as the distance beween the cellophane and your eye becomes more (and the environment interferes with it more). If you use a polaroid filter coloured red instead, you will see that the colour does not fade as you move your head away ... because the light travelling from it is in parallel straight lines. (Well, only if there is no other light apart from what is coming from the pastel-coloured paper) The way the colour fades will be different depending on the environment you are doing the experiment (or thought-experiment) in.

By the way chris, just curious, how old are you?
 

Offline CosmicAudioChic

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Cling film and colours
« Reply #7 on: 28/01/2009 04:10:34 »
;)  Light travels in a straight line and refracts when it passes from one substance to another, which is the principle behind why a prism separates white light into individual colors. Colors seen by the eye are a result of light being reflected, not absorbed.
« Last Edit: 28/01/2009 04:13:41 by CosmicAudioChic »
 

Offline daveshorts

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Cling film and colours
« Reply #8 on: 28/01/2009 11:37:30 »
What exactly is the effect we are trying to explain?
 

Offline LeeE

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Cling film and colours
« Reply #9 on: 28/01/2009 12:41:39 »
If the question is referring to an apparent darkening of the colour of the paper, it's due to reflection.

When you look at a piece of non-reflective paper the light that you see is the light that illuminates the paper after it has been scattered by the paper's non-reflective surface; because the light is scattered pretty evenly across the surface of the paper, the corners will be as bright as the center.

When you look at a piece of reflective material though i.e. the matt paper covered with cling-film, you'll only see the light that's directly reflected back to your eyes; look at a light-bulb (a dim one for safety reasons) in a mirror; the reflection of the light bulb itself will be very bright but the area around it, where the light bulb isn't reflected, will be much darker.  When you then replace the mirror with a plain piece of paper, the sheet of paper will be more or less evenly illuminated.
 

Offline yor_on

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Cling film and colours
« Reply #10 on: 28/01/2009 14:14:42 »
A guess:)

What we call colors are just different wavelengths that our eyes have learnt to interpret.
"cellophane transmits light throughout the region 400--700 nm, with. maximum transmittance between 500--560 nm"

If you place 'colorless' cellophane over something colored it should get darker as the wavelengths will interact with the atoms inside that cellophane and therefore lose some energy in the 'interaction'.
As well as they will be refracted and dispersed from their original paths.
I doubt that cellophane is a 'perfectly transparent medium' though:)

So you should get a loss of energy due to the cellophanes interactions with the 'original' photons getting 'exchanged' (particle seen that one:) as well as dispersion.

And depending on the angle of the cellophane there might even be a color change?
It's basically a oil based material right?
Anyone seen the rainbows forming 'over/in' a oil slick.

And it polarizes light.
Here is a really cool photo of that.
http://www.flickr.com/photos/greenspice1964/3159587933/

And a cool 2D->3D idea
http://www.trnmag.com/Stories/2003/082703/Cellophane_turns_LCDs_3D_082703.html

"Light which has its electric vector oriented in a predictable fashion with respect to the propagation direction. In unpolarized light, the vector is oriented in a random, unpredictable fashion. Even in short time intervals, it appears to be oriented in all directions with equal probability. Most light sources seem to be partially polarized so that some fraction of the light is polarized and the remainder unpolarized.

According to all available theoretical and experimental evidence, it is the electric vector rather than the magnetic vector of a light wave that is responsible for all the effects of polarization and other observed phenomena associated with light. Therefore, the electric vector of a light wave, for all practical purposes, can be identified as the light vector..........

.......A third mechanism for obtaining polarized light is the Polaroid sheet polarizer, of which there are three types. The first is a microcrystalline polarizer in which small crystals of a dichroic material are oriented parallel to each other in a plastic medium. The second type depends for its dichroism on a property of an iodine-in-water solution. The iodine appears to form a linear high polymer. If the iodine is put on a transparent oriented sheet of material such as polyvinyl alcohol (PVA), the iodine chains apparently line themselves parallel to the PVA molecules and the resulting dyed sheet is strongly dichroic.

A third type of sheet polarizer depends for its dichroism directly on the molecules of the plastic itself. This plastic consists of oriented polyvinylene.     

(--that third one--- would that be Cellophane????
dichroism means angles of a crystal, so that it exhibits two different colours when viewed from two different directions-)
"

Anna, explain thyself:)
Are you saying that cellophane can/will 'split' the wavelengths?

How can one explain polarizing using a 'particle explanation' btw?

------------------------------------------

Looking at Anna's 'original' date of question...
SH*** :)

But a lovely name that one...
With a whiff of summer.
--
Is this a quiz?
« Last Edit: 28/01/2009 15:15:24 by yor_on »
 

lyner

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Cling film and colours
« Reply #11 on: 28/01/2009 22:19:13 »
This thread has ranged far and wide and I will just try to sum up the relevant bits.
Filters and paints work by selectively absorbing some wavelengths and letting others pass through or be reflected.
Coloured cellophane is not a very 'good' filter. It may let unwanted wavelengths through. 5 to 10% of unwanted wavellengths could get through.
If the cellophane is placed directly on the card then light has to pass through it twice before you can see the effect - in, from the light source and out, towards your eye. so it has twice the effect. Unwanted wavelengths will be reduced by 10% of 10% - which is 1% so the colour will be much more 'saturated'.

One layer of clear cellophane will not have much of an effect; you may lose a few % of the light energy. That is unless it is actually in contact with the card. The effect could then be similar to wetting the surface. There will be less light reflected at the surface and more will be reflected after penetrating the upper few layers of atoms which will absorb some of the unwanted wavelengths and reflect others. The wavelength selectivity of the pigment will be higher because less 'white light' will be reflected by the surface - so the colour will look 'stronger' - more saturated.
 

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Cling film and colours
« Reply #11 on: 28/01/2009 22:19:13 »

 

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