Microscope Forensics

When investigators are at the scene of a crime, they look for the smallest pieces of evidence, and then analyze them under a microscope in the lab. Some of the things they might look at are fibers from clothing or carpet, hair, soil tracked in on shoes, fingerprints, and blood stains. Those don't seem like they'd tell you much, but under a microscope, surprising details come to light.

Materials

• Compound microscope
• Magnifying glass
• Plain glass slides
• Paper and #2 pencil
• Samples of hair from people and animals
• Several different types of fibers (wool, silk, rayon, etc.)

Fingerprints

Why do the police use fingerprints to help identify criminals? For one thing, it's evidence that is easy to find - we leave prints behind us every time we touch a smooth surface without gloves. The amazing thing is that every person's fingerprints are unique. There is no one else who has the same fingerprint as you! When prints are left behind at a crime scene, police first have to show that they weren't left behind by someone who was supposed to be there. If your house was robbed, for example, they would probably take fingerprints from everyone in your family to help them identify and rule out all the prints that didn't belong to the thief.

1. Make a fingerprint library of you and your family members. One easy way is to rub a newly sharpened pencil across a piece of paper about 20 times until you have a nice layer of graphite (pencil lead). Then rub your fingertip in the graphite and press your finger down on a piece of scotch tape. Then you can stick the scotch tape down on a fresh piece of paper and label which finger it was a print from.
2. Look at your fingerprints with a magnifying glass. What kind of pattern do you see? The most common patterns are called arch, whorl, and loop; go here to see examples of fingerprint patterns.
3. Your prints may look very similar to someone else in your family. Try taking a print and sticking the tape on a glass microscope slide and use a microscope to look for tiny differences between the prints.

There are two types of fingerprints, visible and latent. Investigators use two different methods to lift these prints from a surface. Learn how to use both methods in our Forensic Science Experiments article.

Hair

Hair can be valuable evidence. Looking through a microscope, investigators can perhaps tell what ethnicity the criminal is, or whether the hair has been dyed and how recently. Find several different hair samples to look at, and make sure to include some from your pets so you can see how animal hair is different from human hair!

1. Put a strand of hair on a slide and hold it in place with two small pieces of tape. (You can also make a wet mount by placing  a drop of water over the hair and covering it with a coverslip.) Compare different colored hair, if you can. One difference is that brown hair will generally be thicker than blond hair.
2. Look at it under the microscope, starting at low power and working your way up to higher magnification. Since hair is not a flat object, you probably won't be able to focus on the entire strand at the same time. Adjust your focus to see the outside edges of the strand clearly, and then adjust it again to look at the part in the middle. Do you see a rough, scaly surface?
3. Try it again with cat or dog hair. Are they scaly too? Do their scales look like human hair, or are they different? Draw the differences.
4. Have a family member give you a piece of unidentified hair and see if you can tell whether it is human, cat, or dog.

Hair can also be used for a DNA test if the strand left behind at the crime scene still has the follicle (the part that attaches to the scalp).

Fibers

Was the criminal wearing a wool sweater or a polyester track suit? You can find out by looking at cloth fibers under a microscope! Wool fibers have very distinctive scales, whereas man-made fibers like nylon or rayon have a very smooth, regular shape.

1. Gather samples of several different types of fibers like wool, cotton, rayon, or silk. You don't have to ruin your shirt to do this! Even just a tiny strand trimmed from the fuzz will work, or look for loose threads.  Place your sample on a slide, put a drop of water over it, and cover it with a coverslip.
2. Look at the fibers under the microscope and draw pictures to help you distinguish between them. Are they smooth or scaly? Are some thicker than others?
3. Have a family member give you a "mystery fiber" and see if you can identify it by comparing it with the samples you prepared yourself.

Go to this site to learn more about different kinds of fibers and to see pictures of them taken with an electron scanning microscope!

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