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I was stretching a pink colored rubber band, and I noticed that the longer I stretch it, the lighter the pink becomes.

Why does this happen?

wavion
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    I just comment because I am not sure about details but scattering does play a role. It can be on the size and shape of polymer micro-domains as in an answer below, or simply by modulating the interdistance between segregated pigment or other loads domains. Moreover if the change is on the surface, but still no light is passing through, fissures on the surface would scatters giving the whitish color. Ps also mentioned in an answer. – Alchimista Apr 18 '20 at 12:03
  • Hey wavion, I just saw this question has a bounty. What about my answer is not sufficient for you? – BioPhysicist Apr 22 '20 at 21:19
  • Hi Aaron. I got the answer I was expecting just now. It seemed to me that transparency and pigments doesn't fully explain what happens to the rubber band. If you check Gert's comments on Anders Sandberg's answer, it doesn't seem to be completely convincing. When I read your answer, I was left a little confused about which exact optical properties were being referred to here and how they worked. Krishnanand J's answer was a little more comprehensive, and showed exactly how the rubber band behaves. – wavion Apr 23 '20 at 04:13
  • Thanks a lot for your answer though! I see after reading Krishnanand's answer that it was indeed a correct explanation. I was just looking for a more, if you will, toddler-like explanation of this property that could also clear up the points raised in the comments of Anders Sandberg's answer. – wavion Apr 23 '20 at 04:15
  • I'm glad you got the answer you were looking for. Next time you are more than welcome to comment on my answer if you are confused about something. Also, make sure to tag me in comments so I get pinged on posts that are not mine. Like @wavion – BioPhysicist Apr 25 '20 at 16:41
  • @AaronStevens, sure thing, thanks a lot! :) – wavion Apr 25 '20 at 17:32

6 Answers6

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Colour can come from pigment particles embedded in the translucent rubber matrix absorbing light. When you pull the band the particles become separated by a longer distance, but being themselves inelastic remain the same size. Hence the amount of absorption per unit area decreases, and the band become lighter in color.

Simulated rubber band Simulated rubber band with pigment particles embedded in the matrix. As it is extended it becomes more translucent

Rubber bands are also incompressible ($\nu=1/2$) so the volume is largely unchanged by pulling. This has the effect of reducing the cross section, further reducing absorption.

Anders Sandberg
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    I'm not convinced by your explanation. Rubber is an incompressible material: it's volume doesn't change when you stretch it, so the average difference between pigment particles doesn't change on stretching. – Gert Apr 16 '20 at 21:00
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    @Gert, the volume may not change but the density of the pigment particles definitely decreases. It’s analogous to crumpling up a dotted paper. Along the plane of the paper the dot density is fixed. But the volume dot density is definitely higher than when the paper was uncrumpled. – Superfast Jellyfish Apr 16 '20 at 22:02
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    Same number of pigment particles in the same volume means equal density of pigment particles. Or take a cube of blue playdough (or Bluetack). Now re-mould it into a long square bar: the colour won't change because the pigment (or dye) density doesn't change on re-moulding. – Gert Apr 16 '20 at 23:05
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    @Gert - I added a simulation done in Matlab. The effect is visible. – Anders Sandberg Apr 16 '20 at 23:43
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    @Gert I think Anders is arguing that it isn't about pigment/volume. Its about pigment/surface area. The latter definitely changes when you stretch things. Perhaps the pigment doesn't penetrate through the rubber? – KF Gauss Apr 17 '20 at 00:13
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    Or the optical penetration depth through rubber is much longer than in opaque things like playdough – KF Gauss Apr 17 '20 at 00:19
  • @KFGauss: I think you've got it. "Density" may be the wrong word, at least if used in the formal "per volume" sense. "Areal density" (or "area density") seems to be an existing term for this concept, though. I would say it's not surface area, but cross-sectional area, though. – Tim Pederick Apr 17 '20 at 04:46
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    @KFGauss For most rubber bands the pigment is not on the surface but embedded (cut a pink rubber band and the insides are pink) so surface area is not correct but more like layered 3D sum of surfaces - wait, there's a word for that in everyday speech: volume. Granted the mechanics is not changing density but deforming volume kind of like what happens when you squeeze a half-bottle of coke - the coca-cola is incompressible but the bottle still deforms and becomes thinner at the waist allowing light to make the coke look less dark – slebetman Apr 17 '20 at 05:47
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    @slebetman [...] the coca-cola is incompressible but the bottle still deforms and becomes thinner at the waist allowing light to make the coke look less dark That would work for translucent rubber but MOST rubber isn't translucent at all. And particles of pigment are uniformly distributed throughout the volume of rubber. – Gert Apr 17 '20 at 13:32
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    This answer is proper 'bad science'. Filled rubbers/elastomers aren't translucent at all. I say this as a material development engineer of 10+ years in the rubber/elastomer industry, developing client oriented solutions (formulations). I will shortly be executing some experiments with pigmented rubbers myself. – Gert Apr 17 '20 at 13:45
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    @Gert could you possibly write an answer explaining this phenomenon? Hopefully your expertise could be of assistance... – wavion Apr 17 '20 at 14:48
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    @Gert: Rubber may be incompressible, but it certainly is deformable. Stretch a rubber band, and it becomes thinner and narrower. Use a pen to put some dots on the band at a given distance apart before you stretch it. After it's stretched, won't those dots be further apart? – jamesqf Apr 17 '20 at 16:58
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    I don't think this answer is correct. When I test this on a real-life rubber-band, the color changes suddenly and drastically as I get close to the snapping point, not slowly and linearly as I would expect with this explanation. The discoloration also forms a triangular point near the curve of the band (with the center darker than the edges), rather than a single color or a continuous smooth gradient as this answer would suggest. – BlueRaja - Danny Pflughoeft Apr 17 '20 at 18:53
  • @Gert I was assuming that the pigment density is uniform in the stretched polymer (at the molecular level). This then implies greater density when the polymer is tangled up. Am I getting something wrong? – Superfast Jellyfish Apr 18 '20 at 06:13
  • Microvoid formation -- same as why plastic turns white when you stretch it. – Matt Sep 01 '21 at 11:33
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Rubber bands are made of polymers (more specifically elastomers). A given polymer in the band can either be aligned with other polymers around it, or it can be misaligned. Therefore, you can end up with regions of order and regions of disorder in the band. In an unstretched band you have much more disorder, but when you stretch the rubber band you are forcing the polymers to become more ordered and aligned$^*$. It is this alignment that changes the optical properties of the band, causing it to scatter light differently and appear to be more white.

Relating this to Anders Sandberg's answer, unstressed the rubber is more transparent, but stretched the rubber is more opaque, thus causing fewer pigments to be visible.

$^*$This also explains why heating up a rubber band causes it to shrink, as the extra energy causes the polymers to become less aligned, which causes the band to decrease in length.

BioPhysicist
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    Is there a straightforward way to see why the stretching scatters light to make it look more white? This seems to be a common property of rubber bands of all colours. Is it that light now is less trapped? – Superfast Jellyfish Apr 16 '20 at 21:56
  • so, the states in grand canonical ensemble were shifted to higher energy states, thus the amount of quanta required to kick the electron changed, and thus change the frequency of light? (Also, shouldn't entropy increase with the given disturbance, as potential and work done on the system increase?) – ShoutOutAndCalculate Apr 17 '20 at 07:13
  • @ShoutOutAndCalculate I don't think light scattering is the same as absorption/emission of photons due to transitions between atomic energy levels – BioPhysicist Apr 17 '20 at 13:19
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Stretching rubber makes it shiny

enter image description here

Image credits: onelittleproject.com

We know, balloons become highly reflective after inflation. This effect applies to rubber bands too. Rubber has a highly coiled structure, which scatters light randomly, giving it a soft appearance.

enter image description here

Image credits:Balloon science

Upon stretching, the coils unwind and reflect more uniformly, which gives it a shiny look. This answer and this answer elaborates this effect well.

So, on stretching a rubber band, it becomes more reflective, which means more light would reach your eyes, giving a perception of a lighter colour.

Presence of surface impurities

It may not be relevant, but the presence of surface impurities affects the colour change. In real life, rubber bands tend to gather dust and other contaminants over time. For ancient rubber bands (like the one I have) there is a complete layer of dark coloured impurities coated on its surface.

enter image description here

Here are some close-ups. You can see that the surface impurities are more spread apart in the stretched band.

enter image description here

Stretching the rubber band exposes the colour of 'fresh' rubber inside. This answer offers a good explanation.

Here, I would like to emphasise the reason why I haven't considered the transparency effects.

Transparency does not always imply 'lightness'

A stretched rubber band is indeed more transparent than an unstretched one. But does that make the rubber band lighter? It does, only when you view it against a brighter background.

enter image description here

As you can see, even in the dark background, the stretched rubber band is still lighter than the unstretched one. This shows that for my rubber band, the reflectivity effects are predominant over the transparency effects. This may not always be true and depends on various factors like the thickness of the band, type of rubber and pigment density.

Transparency effects are much more noticeable in thinner objects, like rubber sheets. Interestingly, this effect can be exploited for making smart windows, that can precisely control the amount of light passing through it.

enter image description here

Credits: MIT

AlphaLife
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In general the rubber from elastic bands scatters light. This explains why rubbers are opaque, but still reflect light quite a lot, in the same way as milk looks white.

In the presence of pigments, the whitening when pulled can be explained if the material scatters light more when it is stretched. More scattering means that light rays that get reflected, or scattered, at the surface of the rubber have to travel a smaller distance through the material before getting reflected. Hence, they have less chance to hit a color pigment. (Pigment density remains constant, as rubbers are fairly incompressible.)

The remaining question is: Why is scattering larger when stretched?

From this source:

From these, it is concluded that:

(a) Optical heterogeneities are present in stretched samples having a size of the order of several microns.

(b) These heterogeneities are not present in the unstretched samples and develop with elongation in the same (or lower) elongation range where crystallization occurs.

[...]

(d) The heterogeneities primarily represent fluctuations in the orientation of anisotropic bodies rather than fluctuations in density. [...]

Aaron was on the right track by mentioning scattering, but it seems that heterogeneities in the material are more relevant than crystallization to explain the whitening by light scattering.

Roan
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Anders Sandberg explained the main effect (in fact, decreased per-area pigment concentration and the band becoming thin are the same effect).

There are two more effects that acts even for non-translucent rubbers, making them whiter:

  1. The presence of tiny cracks on the surface. When stretched, they open wider. One can see the same effect on macro scale in aged rubber. The cracks allow more light to scatter instead of going deep in the volume of the material and being absorbed (the same effect makes the snow white and the deep water black, blue or whatever less white).

  2. The presence of tiny voids in the bulk of the rubber, containing filler, pigment, air or just a discontinuity in the polymer. When subject to negative pressure, these also expand and scatter the light more than in non-stretched state.

Community
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fraxinus
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    You can see effect #1 in many materials, for instance repeatedly bending some metals until they start to fatigue. – jamesqf Apr 17 '20 at 16:54
  • Like the tiny crack. If not transparency is involved, scattering by the surface is certainly at play. – Alchimista Apr 18 '20 at 12:04
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It is very important to understand that contrary to popular belief, Sunlight is white light (not yellow).

Now you are asking why does a rubber band become lighter colored when stretched, and your question can be why does a rubber band refract/reflect more light when stretched without changing too much the (distribution of) energy levels of the photons, more specifically why does the refraction/reflection contain all wavelengths of photons (thus combining into more and more whiter/brighter light).

Now the answer is crystal lattice structure. There are certain materials that have this kind of molecular structure, like glass, crystals, and certain plastics, and they have the ability to refract and reflect light (visible light too) without changing too much the (distribution of) energy levels of refracted photons and more specifically able to refract/reflect all wavelength (visible too) photons and absorbing very little.

enter image description here

http://www.schoolphysics.co.uk/age14-16/Matter/text/Rubber_band/index.html

Now what happens to rubber bands as it gets stretched is that the molecular structure of the rubber is able to change, and it changes towards these crystal lattice structures of certain materials (glass, crystals and certain plastics), and thus is becomes able to refract/reflect more light without changing too much the (distribution of) energy levels of the photons, more specifically, it becomes able to refract/reflect all wavelength (visible too) photons (thus able to seem colored more like the light in the environment).

It is a very good example, if you try to stretch a rubber band in a room filled with dark blue light. When you stretch the rubber band, it starts to refract/reflect more dark blue light, and in fact you could see the rubber band seem to becoming darker (not brighter).

Another very good example is the balloon, try to fill the balloon with air and see as it seems to become lighter in color, but do it in a dark blue light filled room, and the balloon might seem to become darker as you fill it with air.

Árpád Szendrei
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    Actually, relative to the sRGB white point, sunlight is yellow. In particular, its chromaticity is $(x,y)=(0.326, 0.338)$, which yields sRGB #fff3ea. For more details, see [What color is the Sun?
    • chromaticity above the atmosphere](http://www.vendian.org/mncharity/dir3/starcolor/sun.html).
     – Ruslan Apr 18 '20 at 19:28
  • without changing too much the energy levels of the photons - I think you mean without changing the distribution of energy levels. A normal pigment works by absorbing some wavelengths more than others, not by changing the energy level of any specific photon it scatters. (A material that fluoresces could do that, e.g. making a starched shirt look "whiter than white" for a given ambient light, or some day-glo neon colour.) Anyway, I know that the normal pigment mechanism is selective absorption vs. scattering, but your answer could be misleading for people that don't. – Peter Cordes Apr 19 '20 at 17:54
  • @PeterCordes correct thank you I will edit. – Árpád Szendrei Apr 19 '20 at 18:37
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    @Ruslan: "white" means "having a broad spectrum" rather than a particular perceived color. – Ben Voigt Apr 19 '20 at 20:13