Make colours appear in adhesive tape, but only when you view it in a certain way. Like magic ink. This is a phenomenon that can be explored by scientific inquiry and it is possible to make a model (a set of rules on how the effect is produced) that can be used to predict the outcome. This is the core of what scientists do and this is an example.

The goal is to let you explore the phenomenon yourself and reach your own conclusions. These can be tested at different points during the process. Should you need help, the material can provide hints and suggestions on how to proceed, but we strongly recommend to do as much as possible by yourself.

You will need:
- Colour LCD monitor (it is important that it is LCD, due to its principle of operation).
- Polarization device (polariser/polaroid, or home-made).
- Transparent adhesive tape.
- Knowledge about the RGB colour scheme.
- Knowledge about polarisation.

There are short tutorials about the topics, but to be able to focus on the main task: the development of the model, you should be familiar with these subjects in advance. We recommend that you return to this material when you are.

How to construct a polarisation device?

Brief tutorial on the RGB colour scheme

What is polarisation?

Put stripes of adhesive tape on a piece of glass so that they are all in the same direction and at least partially overlapped. Make at least six layers, more is preferred.

Place the item between yourself and the monitor.

Look at it through a polarisation device. Turn the device and/or the item around the common axis and observe.

What do you see?

Now it is time to start exploring the phenomenon. From now on, you should try to come up with ideas, how to get information about cause-effect relationships and what those relationships might be.

What do you think the colour depends on, meaning what would you have to change to get a different colour?

The factors that determine the colour of adhesive tape are: thickness (Fig.4), orientation (Fig.5) and inclination (Fig.6).

Fig.3

Fig.4

Fig.5

There are numerous factors that determine what colour we will see. So far this material focuses on one of those. It is planned to be further developed to include all of them.

From now on, perform experiments in such a setup, that you have the polariser in a parallel(transmissive) position with regard to the monitor, and the tape turned so that the colours are most intense.

Why such a setup?

Now it is time to determine the relationship between thickness and colour. Once this is determined, you will be offered suggestions on how to test it.

You have noticed that in the mentioned setup, adhesive tape appears coloured. You can see this again. By clicking on the "Blank" button, you will see a blank screen.

Blank

How would you proceed? (Write important words.)

Since the monitor produces all colours by mixing the three basic colours, each colour is determined by how much of each of the basic colours is present. So, if one blocks all red from white, the result would be a green-blue mix, called cyan. If one blocks a little of green, too, the colour will be darker and more blue than green.
So if we know, how much of each of the basic colours is present at each thickness, we can calculate the resulting colour with any program that can mix colours in the RGB scheme (e.g. Paint).
It would be therefore reasonable to proceed so, that we produce the background in each of the basic colours and see what pattern we see in the adhesive tape. That should allow us to estimate, what percentage of the original colour is transmitted through the adhesive tape at different thicknesses.

Choose the colour of the background.

White
Red
Green
Blue
Cyan
Yellow
Magenta

Do you see any pattern?
(Such as the tape being bright at almost exactly every third layer, or shades changing from bright to dark repeatedly (periodically).)

Back to choosing colours

Yes, I see a pattern

No, I do not see any pattern

It is unusual that you would not see a pattern for any of the colours. We can not, however, exclude the possibility of such a case. We suggest that you use the most regular, probably cheapest, adhesive tape and retry, or retry with some other type.

Usually the pattern for red is most obvious. Click on the button below to learn what the pattern for red usually is, if you can not see it after several attempts.

What is usually the pattern for red?

I do not see a pattern, but proceed anyway

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What pattern do you see for red? Describe it in percentage of the initial brightness for each layer, for the first six layers.

1 layer:
2 layers:
3 layers:
4 layers:
5 layers:
6 layers:

Can you determine what is the period of repetition for red?
(The number of layer after which the pattern is (more or less) repeated.)

What pattern do you see for green? Describe it in percentage of the initial brightness for each layer, for the first six layers.

1 layer:
2 layers:
3 layers:
4 layers:
5 layers:
6 layers:

Can you determine what is the period of repetition for green?
(The number of layer after which the pattern is (more or less) repeated.)

What pattern do you see for blue? Describe it in percentage of the initial brightness for each layer, for the first six layers.

1 layer:
2 layers:
3 layers:
4 layers:
5 layers:
6 layers:

Can you determine what is the period of repetition for blue?
(The number of layer after which the pattern is (more or less) repeated.)

Determine a rule that connects number of layers (or thickness) to transmissivity (how much light passes through). The rule you determine is called a model.

A model must be written so that it enables you to make predictions (like calculate which colour you will see at 7 layers (or more).

Use all that you have discovered so far to build your model.

A little help, please.

You have determined a model to calculate the colours.

You can now test it.

The next slide will provide a number of options to calculate and display a colour, based on values of red, green and blue. This values must be imputed by you (the user) or calculated from a model (if it is in the form of an equation/formula). The program will then display the colour.

How do you test a model?

To test the hypothesis, one might calculate all three basic colours for each thickness and then use a program that can mix colours (e.g. Paint). This is very, very time consuming. There are ways to write a program, that can display all the colours for various thickneses at once.

We wrote a program in MS Excel. It's up-side is that the formula is written in an Excell cell, so any user, familiar with Excel, can change it. It's down-side is that MS Excel is not freeware and that the file uses macros to display the resulting colours (which means, you have to enable macros in Excel to use it). At this time it has not been tested on OpenOffice.org Calc.

We wrote another program using HTML and JavaScript. Its up-side is that it only uses internet technologies, so it can be viewed and used in any contemporary browser. Its down-side is that the formula to calculate the colours is written in the JavaScript code, so, first of all, it is the correct one, and second of all, to change it, one must change the code itself.

The safest and most reliable way is to combine the two. We wrote an Excel file that calculates the colour values based on equations that can be entered in Excel in the usual way. Then it generates HTML code that the user has to copy to a text editor (like Notepad, not Word) and save as *.html. The html file then displays the colours. There are no macros here and OpenOffice.org Calc can probably also be used instead of Excel (not tested).

Congratulations!

You have come to the end of what we have prepared. We encourage you to continue exploring this and any other phenomenon you may encounter.

Finish

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You can construct your own polarisation device. This device uses Brewster's angle to achieve polarisation.

Use two pieces of glass or a piece of glass and a (small) mirror. Cover the glass (both if you have two) on one side with black paper or some other matt black material that absorbs most of the light (e.g. cloth). Position the glass pieces in a periscope configuration (see Figure 1). Make sure that the angle of both pieces with the incident direction is 34° (90° minus Brewster's angle, which is 56° for glass). Fix the position of the pieces. You may construct a cardboard box (see Figure 2) or glue them on a surface (see Figure 2). Do not use adhesive tape.

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The sensors for colour vision in our eyes (called rods) are of three types. One is most sensitive to red, one to green and one to blue light. Mixing various amounts of each of these colours of light produces all the colours we can see. Displays exploit this by using the so called RBG colour scheme. RGB stands for Red, Green and Blue. Monitors can emit light with only those colours and mixing them in various amounts produces all the colours they appear to emit. If one looks closely (perhaps with a lens) at a monitor (TV displays are better for this purpose because they have bigger pixels), one can see that each pixel is made of three parts: one red, one green and one blue. To play with colour mixing, use a drawing program (e.g. Paint) and define your own colour. Values of red, green and blue for that colour will be displayed somewhere, usually in values ranging from 0 to 255, 0 being none of that colour and 255 being the most of that colour.

So, what you need to know here is: the only colours that the monitor actually produces are red, green and blue. To achieve, for example, yellow, it mixes red and green, so yellow is not really yellow, but is red and green together.

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Polarisation is a property of light that is successfully used in LCD monitors. It is the basic principle by which they turn on and off pixels. We will compare polarisation to wood in this explanation. The wood analogy will be written in <span style="color:brown">brown</span>.

For purposes of this investigation, we may imagine each ray of light as having a circular shape (<span style="color:brown">a trunk</span>. Such light is non-polarised or circularly polarised. A polarisation device can be imagined as a device containing one slit per ray (<span style="color:brown">two parallel blades</span>). So every ray that passes through a polarisation device is transformed from having a circular shape to having a linear shape (from a cylinder to a thin strip, <span style="color:brown">from a trunk to a plank</span>). Such light is said to be linearly polarised. If this light now passes through another polarisation device oriented in the same direction, nothing happens. The ray still has a linear shape (<span style="color:brown">the plank can pass right between the blades</span>). If, however, this light passes through a polarisation device orienter perpendicularly to the first, nothing passes through. <span style="color:brown">Like trying to make a plank from a plank positioned perpendicularly to the saw blades. One would get a thin rod. If the plank was really thin (like veneer), then the rod would be barely visible compared to the initial trunk.</span> The more the orientation of the second polarisation device is parallel to the first, the more light is transmitted. <span style="color:brown">The more the blades are parallel to the plank the less of the plank is cut off.</span>

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There are devices and materials that can turn polarisation (<span style="color:brown">twist the plank</span>) or even produce circular polarisation from linear (<span style="color:brown">transforming the plank back into a trunk, or, more correctly, turning the plank about its longitudinal axis so fast that it appears to be a trunk (circularly polarised), and it behaves like a trunk for purposes of polarisation</span>).

This is only a fast glimpse at what polarisation is. We strongly recommend that you first familiarise yourself with what polarisation really is and then return to this material, so you can make full use of your knowledge to form a correct model.

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