Tuesday, 31 December 2013

Make your coins green!

  1. Half cup of lemon juice
  2. One tablespoon of table salt
  3. A drinking glass or a transparent jar
  4. Some dirty pennies or coins made of copper or copper coated (Such as American pennies)

  1. At first, mix the lemon juice and the table salt inside the drinking glass or a transparent jar.
  2. Then add a few dirty copper coins (Such as American pennies) or just pieces of copper inside.
  3. After that, let those coins stay in the jar for about five minutes.
  4. Then take those coins out and let them dry on a dry paper towel or any clean plain surface.
  5. Leave those coins or copper pieces to dry for overnight.
  6. Examine those dried coins the next day and record any changes that appear on them.
Something green and solid should cover the copper coated coin.

In the article “Make your coins as shiny as new!” we used lemon juice to clean dirty old coins. But in this experiment, the dirty old coins get covered in green coating.
Now the question is, how can the same chemicals produce completely two different results?
The last step of “Make your coins as shiny as new!” experiment was washing/rinsing the soaked coins in running water. But in this experiment, we won’t wash/rinse those coins. Now that's what creates the main difference in these two experiments. Instead of washing, the mixer of lemon juice and table salt are allowed to dry on the coins. Copper can react with the oxygen to form copper oxide. When the copper coated coin gets old, the copper coating of the coin reacts with the oxygen of the air and produces copper oxide.

CuO (Solid) + H^+ ----------> Cu^2+ + H20
Copper oxide + Hydrogen ion ----------> Copper ions + Water

When lemon juice-salt mixer is allowed to dry on such a coin, the mixer simply dissolves the copper oxide. But without the washing/rinsing, another chemical reaction takes place.

The copper atoms reacts chemically with oxygen from the air again and at the same time also reacts with chlorine from the salt. Reaction with both the oxygen and chlorine creates a blue-green compound. This compound is called Malachite.

Malachite is one of the oldest known green pigments mankind has ever known. More than six thousand years ago ancient Egyptians used to use malachite as a makeup material. Around AD 900, the ancient Chinese began to use malachite. They used ground malachite mixed with water to make painting colors for painting.

  1. Try this experiment by not adding salt into the lemon juice.
  2. Try using vinegar to shine the copper coins.
  3. Try other kinds of less powerful acids to do the experiment.

Color changing fish!

  1. A black marker or a black sign pen
  2. Red, green & blue (RGB!) construction paper
  3. 4 sheets of white pape
  4. Scissor
  5. Glue 
    [All the materials can be obtained from a general store or a convenience store]
    Red Paper Fish

  1. At first put the red, green and blue construction one over another.
  2. Now draw the outline of a fish on the topmost construction paper.
  3. After that, use the scissor to cut the outline of the fish from all the construction at once. So now we have 3 paper fish with red, green and blue colors.
  4. Use the black marker or sign pen to draw the eye of the fish on each cutting. Now we have 3 paper fish.
  5. Glue each paper fish onto different pieces of white sheet. 3 out of 4 white sheets are used. So now we have only 1 white sheet.
  6. On the last white sheet let’s draw a large fishbowl. Large enough to fit one of our paper fish inside the drawing.
  7. Lay one paper fish and the fishbowl drawing next to each other on a plain surface.
  8. Now stare at the paper fish’s eyes for about 20 to 30 seconds. Then quickly stare at the fishbowl drawing.
  9. Record what you see.
  10. Now repeat the same process for the other two paper fishes and record your observation. 


  1. After staring at the red paper fish you should see the shape of a blue-green colored fish in the drawing of the fishbowl.
  2. After staring at the green paper fish you should see the shape of a magenta colored fish in the drawing of the fishbowl.
  3. After staring at the blue paper fish you should see the shape of a yellow colored fish in the drawing of the fishbowl.


In the article “Black & white - Source of colors!(BENHAM’s disk)” we were able to learn that in the retina of our eye there are some special cells called cones which are responsible for vision. In this experiment, the imaginary image which we see on the white paper after staring at any colored paper is called an afterimage. It’s called afterimage because it appears after the real stimulus is removed. This case of afterimage results when our cones gets tired.

For example, when we look at the red paper fish, the image falls on one part of the retina. The red sensitive cones in that area gets tired over time and stop responding to anything. So after that when we turn to look at the white paper, the red sensitive cones are too tired to respond to anything by then. Our eyes only see white when all three types of cones respond equally. So, when the red sensitive cones gets tired, it creates an imbalance and a color other than white is seen. The only cones responding at that time are the green and blue cones. That means the fish afterimage we will see should be blue-green color. Similar events occur with the other colored paper fishes.

Use other colored paper fishes to see the effects and record them.

Sunday, 29 December 2013

Black & white - Source of colors! (Benham’s disk)

Benham's disk

  1. An old disk/CD/DVD. (Which you don’t want anymore)
  2. Glue (Can be obtained from a general store)
  3. Scissor (Can be obtained from a general store)
  4. White sheet (Can be obtained from a general store)
  5. Black marker (Can be obtained from a general store)
  6. A rotator (For this you can use things such as – electric motor, electric screwdriver, turntable, hand drill or maybe a toy top.) (Can be obtained from places such as electronic shop, hardware shop, toy shop etc.)
[Warning: Be careful if you are using an electric screwdriver or a hand drill.]

  1. Use the scissor to cut the white sheet to give it the same size of the disk (CD/DVD).
  2. Use the black marker to color half of the circular white sheet into black. You can fold the circular white sheet so that you can have two equal sides. Then you can color one side which will be half of the sheet to make it black & white.
  3.    Now glue the circular white sheet to one side of the disk. Now the disk can be called the name “Benham’s disk” which is obviously.
  4. Now you have to use some kind of rotator to rotate the black & white Benham’s disk. To rotate the disk the top of the rotator must be attached at the middle point of the disk (CD/DVD). You can place the disk on a turntable, small electric motor, electric screwdriver, hand drill or some kind of toy top. You can use glue to attach the disk on top of a rotator.
  5. Now make the disk rotate. Make sure that there is a lot of light around.
  6. Now observe what happens!

  1. Colors will appear as the disk spins.
  2. The rotation speed of the disk will impact the appearing colors.
  3. Different people may notice colors of different intensities.
Rotating Benham's disk

The retina of our eye is responsible for sensing or detecting light and color. There are two different kinds of cells in the retina. They are – rods and cons. Rods help us see in low light conditions. Cons helps us to see in bright light. Cons are also important for color vision.
There are three types of cons altogether. Each cone can sense or detect different colors of light. These colors are – RED, GREEN & BLUE. Each type of cone needs a different amount of time to respond to a color. Furthermore, the cones continue to respond to a color stimulus even after it is gone! But they differ in the amount of time they do so.
We don’t totally understand what happens when we look at the Benham’s disk. But one theory about it is that the black & white areas stimulate different parts of the retina almost at the same time. The white area of the disk activates all three types of cones (Because white is the reflection of all colors in a certain ratio!). On the other hand the black area of the disk deactivates all three types of cones (Because black is the absorption or absence of any or all kinds of colors!!).
When the Benham’s disk is rotating, the black & white side switch places simultaneously and this happens really very fast. So a series of quick cone activation and deactivation takes place in our eyes retina. The activation and deactivation happens so quickly that the nervous system of our eye can’t keep up with it and gets confused. This results an illusion of bands of colors on the top of the rotating black & white disk.
It may show different color bands at different rotating speed, different brightness of light, different kinds of light etc. Different people will see different kinds of colors watching the same disk at the same time because everyone’s eyes have a different distribution of cones on the retina of their eyes. The sensitivity of their cones are also different.


In the year 1894, C.E Benham, a toy maker mounted black and white patterns on spinning tops. He marketed those spinning tops under the name “Artificial Spectrum Top”. However, the effect was first discovered in the year 1838 by Gustav Fechner. That’s why the colors which can be seen on the rotating black & white Benham’s disk is often called or referred to as Fechner’s colors.

  1. Use different kinds of rotating speed to see the color effects.
  2. Use different kinds of lights to see the color effects.
  3. Look through different kinds of lances to see if anything happens.

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