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Finding Fingerprints

In this activity, students learn how to dust and lift latent fingerprints from an object and then study the resulting impressions. 

We leave impressions, or prints, with skin ridge patterns, on everything we touch. Impressions can be of a palm, foot, face, or even an elbow, but the most common are fingerprints, since humans love to make good use of their opposable thumb.

Fingerprints are what we call the pattern of ridges on the fingers that provide a rough surface that helps create the friction needed to pick up a baseball or hold onto a pencil; they are also know as "friction ridges". These ridges are formed before birth, one of the deepest layers of skin pushes upwards, making ripples in the layers of skin above it. By the time a baby is born, there are seven layers of skin, and fingerprint ridges ripple through the top five layers. 

Baring loss of a finger tip to injury, or intensive scarring, the friction ridge pattern of individuals will constantly re-grow the same as their skin re-news throughout their lives.

Dactyloscopy (fingerprint identification) is useful to forensic scientists when they compare two fingerprint samples to determine whether or not they came from the same person.

Fingerprints can be visible when fingers transfer materials like dirt, blood, ink or oil to another surface, but we leave invisible (latent) fingerprints, too.

Latent prints are made by sweat or other residue present on our finger ridges. Because we can’t easily see latent prints, forensic scientists must process them. One method of processing a fingerprint is by dusting for them. This is done by coating fingerprints with powder, “lifting” them, and then taking them to the forensic lab where they are compared to fingerprints in the database. 

Collecting latent fingerprints at a crime scene is a very important task for forensic scientists. One of the most important things to consider at a crime scene is who might have touched what. Latent fingerprints can even be taken from surfaces that are not smooth, for example, clothing or paper towel. Even if fingerprints are months or years old, new technology can allow forensic scientists to recover fingerprints from these objects. 

Fingerprints have three main classes of friction ridge: the arch, whorl and loop.

  • Arches have lines that start on one side and rise and exit on the other side of the print. They look like a hill.
  • Loops have lines that enter and exit on the same side of the print. They look like an upside-down U.
  • Whorls have circles that spiral and do not exit on either side of the print. They look like a bull’s eye.

Every fingerprint is unique, but there are certain patterns that can be observed which many prints have in common. Explore student's fingerprints in this activity. Interestingly, even fingerprints from the same individual can vary slightly from finger to finger!

Objectives

  • Explain how invisible (latent) fingerprints are developed and analyzed through physical means.

Materials

Key Questions

  • What are latent fingerprints?
  • Why do fingerprints need to be “lifted”?
  • What types of patterns do you see?
  • Are the fingerprints of one person all the same?
  • Which surfaces are best for collecting fingerprints?
  • Which surfaces do you think would be a challenge to collect fingerprints from?

What To Do

  1. Create a latent fingerprint. Rub one finger along the side of your nose to make it oily, and then press the fingertip to the side of the glass (oily or sticky fingers leave the best prints).
  2. Assume the role of detective and put on gloves to avoid contaminating the evidence.
  3. Apply charcoal powder to the brush, and then gently brush the powder onto the side of the glass so that the fingerprint is coated.

Teahcer Tip: Using too much powder can result in a smudged print. Instead of putting the brush directly into the charcoal powder, shake the closed jar, then load the brush using only the powder that has transferred to the lid of the jar.

  1. Use the brush to gently sweep away the excess powder to reveal the print (where the powder has stuck to the oils left behind by the finger ridges).
  2. Lift the print by gently pressing the tape onto the drinking glass, and then peeling it off.
  3. Stick the lifted print onto the Fingerprint Specimen Collection Card.
  4. Use a magnifying glass to identify the arches, loops and whorls.

Extensions

  • Repeat the process using talcum powder on a dark surface (stick the tape with lifted print to a black sheet of paper) and compare the results.
  • Do the Fingerprint Fun activity as a class to create a class database of fingerprints. Then, create latent fingerprint “evidence” using the fingerprints of 2-3 students (or an adult) in the class. Lift and analyze the latent fingerprints to identify the person..

Other Resources

Science World | Fingerprint Patterns PDF