Experiment of the Week/Trivia

Submitted by rkrampf

To try it, you will need:

- several sheets of writing or printer paper

- a pencil

- water

- a towel or paper towels

Put one of the sheets of paper in the sink and turn on the water.  Get the paper wet on both sides.  Then hold it up and let most of the water drain away.  Lay the paper on a smooth, flat surface, and blot it gently with a towel.

The next step is to place a dry sheet of paper on top of the wet paper.  Then use the pencil to write your "secret" message.  Press down firmly.  When you have written your message, lift the dry sheet, and you will be able to read your message on the wet sheet of paper.

Now wait a minute!  This is supposed to be a secret, hidden message!  Ahh, we just need a little patience.  Let the paper remain on the flat surface until it is completely dry.  Once it is dry, pick it up, and you should find that your message has vanished.  

OK, so the message is gone.  How do we get it back?  All you need to do is get the piece of paper wet again, and the message will reappear.  Why?

Paper is made of fibers of cellulose, usually from wood pulp.  When the paper is wet, the water becomes an optical pathway for some of the light, letting it pass through the paper instead of being reflected away.

The water also causes those fibers of cellulose to swell and spread apart.  The pressure of you writing on the dry sheet of paper compresses the fibers, forcing them closer together, and causing them to shift and become more aligned with each other.  This gives the light an even better pathway through the paper, so the writing will appear darker than the surrounding paper when it is laying on a flat surface.  The writing will also appear brighter than the surrounding paper when it is held up between your eyes and a light source, showing that more light is really going through the compressed paper.

Once the paper dries, the water is no longer there to serve as an optical pathway, so the message vanishes.  Since those fibers remain compressed and aligned, when you add more water, the light pathway is restored, and the message reappears.

Some watermarks are always visible, such as those found on fine stationary.  For those, the image is compressed into the paper as it is being made, so the paper is actually thinner where the image is, causing the mark to be visible, even when the paper is not wet.

Have a wonder-filled week.