Introduction: (Initial Observation)

Introduction: Invisible ink refers to any substance which can be used to write with, which is either invisible on application or disappears quickly, and can be subsequently restored by some means. The use of invisible ink is a form of steganography, and has been used in espionage. 
The simplest forms of invisible ink are lemon juice and milk. Write on paper with a fountain pen, toothpick or a finger dipped in the liquid. Once dry, the paper appears blank. The writing is made to appear by heating the paper, on a hot radiator for example. Invisible inks have been used for secrecy and security for many years. 

The Germans in World War II used invisible inks for secret writings. One suspected spy was found to have large numbers of keys in his motel room. After inspecting the keys it was found that some of the keys were modified to unscrew at the top to show a plastic nib. The keys contained special chemicals for invisible ink! However, codes and secret ink messages were very easily captured and decoded.

Some of the more fun secret writings are concealment messages like invisible inks made out of potato juice, lemon juice, and other types of juices and sugars! Deciphering and decoding messages take a lot of time and can be very frustrating. With experience, strategies, and most of all luck, you'll be able to crack lots of codes and ciphers. 

Fluorescent invisible inks: A substance used as a fluorescent invisible ink is usually a clear liquid. You may use a fluorescent invisible ink to write any thing on paper or almost any other surface. You know what you write, but others cannot see it. A UV light can magically make your writing visible. Most invisible inks can be revealed only once and remain visible for the life of the document. Fluorescent invisible inks however can be revealed as many times as you need under UV light. They will disappear again as soon as you turn off the UV light. This gives a special advantage to fluorescent invisible lights and opens the doors to many new applications for such invisible inks. 

Modern Applications: Invisible fluorescent inks are use to to mark dollar bills and many other valuable documents to prevent counterfeiting. Banks and inspectors of such documents use UV lights to see the hidden marks in order to verify legitimacy of documents. Invisible fluorescent inks are also used to mark other valuable properties for identification and to proof ownership in case of theft. Invisible fluorescent powders and inks are used to detect unauthorized access and theft. These material will remain on hands and clothing of the thief and are viewable under UV light. Clubs and museums are using invisible inks to stamp visitors hands and use it to control re-entry.   

More about UV light: UV or Ultra Violet is an invisible part of light spectrum above blue. UV has a higher frequency (lower wavelength) than visible lights. Although invisible, UV has many of the properties of normal sunlight. UV can cause sunburn, hurt the eyes and even cause discoloration of material dyed with organic dyes. 

Ultra violet light sources can be used at the crime scene or in the laboratory for the visualization of several types of physical/trace evidence. Many human physiological fluids such as blood, semen, saliva and urine may fluoresce under ultraviolet light sources.
Other items such as hairs, fibers, latent fingerprints and even footwear impressions may fluoresce. Examination may also reveal bruising, physiological stains and marks which may not be evident under normal lighting.
(Warning: UV light can damage your retina if the light source is viewed directly.) 

Forensic Investigation using UV light:

Since body fluids like semen, saliva, and vaginal fluids are naturally fluorescent, the use of a light source offers a unique method for locating them. A crime scene investigator can narrow down the specific locations of stains for collection instead of testing entire, large pieces of evidence such as a mattress, a carpet, a sheet, an article of clothing, etc. The dried body fluids will actually glow under the UV light illumination.

Question: What household material can be used as fluorescent invisible ink?

Purpose: The purpose of this project is to identify household material that can be used as invisible ink. 
Many of you may already have such material at home. If you don’t, you will need to expand your search to your local grocery store or hardware store.

Procedure: Do your experiments after dark. Turn off the lights. Turn on your portable UV light. Walk around the home to detect fluorescent material. Make a list of fluorescent objects and material that you discover. Find a way to use those material for secret messages. Inspect all objects, specially look for fluorescent liquids.

Experiment writing secret messages with fluorescent liquids that you discover. Use a fountain pen or toothpick for writing. If the fluorescent liquid that you find has color, use the same color paper to write your message.

Inspect your invisible message under UV light, before and after it dries. Record how long does it take for your newly discovered ink to dry.

Summery of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.