Visible fingerprints can be photographed conventionally but an important scientific problem is how to make 'latent' fingerprints, which are not visible to the naked eye, available for examination. Two scientific approaches are described. The first is the use of chemical change; that is, the transformation of one substance into another, in this case often accompanied by a colour change. The second uses light, and other types of electromagnetic radiation, to interact with the fingerprints and make them visible.

Chemical change:

• Atoms: are the building blocks of all chemical substances. An atom contains three components: negatively charged electrons surrounding a positively charged nucleus containing neutrons withour charge and positively charged protons. The different types of atom are called chemical elements and each element has its own symbol.
• A chemical equation describes a chemical change. A chemical equation has on the left-hand side the formulas of the molecules, atoms or ions that are changed during the chemical reaction: the reactants; and has on the right-hand side the formulas of the new molecules, ions or atoms formed by the chemical change: the products. A balanced equation is one in which there are the same number of atoms of each type on each side of the equation.

Extra resource: Science Bits - Introduction to chemical reactions.

Interaction with light: In fingerprint analysis the observation of fluorescence can be used to visualise some fingerprints that are otherwise invisible to the eye. This technique involves shining ultraviolet radiation onto the sample. Since biological fluids, such as semen, saliva, urine and others fluoresce, and any of these fluids could find their way onto the fingertips that leave fingerprints at crime scenes, an 'invisible' or latent fingerprint may be revealed by fluorescence.

Extra resource: Science Bits - Electromagnetic waves.