Return to The Crime Lab
Introduction: One of the cornerstones of Forensics, is the simple fact that we unconsciously carry evidence of where we've been on our bodies, and we leave evidence of our having been there. Locard said "Il est impossible au malfaiteur d'agir avec l'intensité que suppose l'action criminelle sans laisser des traces de son passage." (It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence.). This is called Locard's Exchange Principle.
"Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value."
- The late chemist Paul L. Kirk, from Crime Investigation, 2nd Edition, J.J. Thornton, ed., 1974, p.2
In this activity an unknown person, who was up to no good, was unconsciously exposed to a material that glows under UV light. Using the portable UV light source, create a list of all the places in the lab portion of the room that glow green.
Now, using the information, trace the pathway of the exposure to account for all of the glowing areas above, and list them, in order of exposure, in the table below: Your task is ultimately to reconstruct the crime committed.
| 1. | 7. |
| 2. | 8. |
| 3. | 9. |
| 4. | 10. |
| 5. | 11. |
| 6. | 12. |
Be sure to DRAW the pathways, using the NUMBERS in the table above, in the diagram of the classroom below.
Questions:
1. When you leave this classroom today, what will you unconsciously take with you (aside from your personal belongings)?
2. What will you have left behind?
3. What are the implications Locard's Exchange Principle to Forensic Science?
CONCLUSION:
Your precinct captain has demanded an explanation as to how the
prints were placed in the room; in other words, you need to recreate the crime
on paper. What happened, in what order, and what was the intent or motive
of the criminal. Your precinct captain hates both extreme brevity, and
excess verbiage, so your report needs to be one page.