How the police collect evidence

There are clues at the scene of every crime. They may be small - Ja fingerprint, a few strands of fibre, a blood stain. But careful searching can give forensic scientists evidence to work with. Much of this work is comparison.

Fibres from the crime scene are collected on clear sticky tape, bagged, and labelled. Later, they can be compared under the microscope with fibres from the suspect's clothing. A match might yield important evidence.

If tiny particles of glass are found on a suspect's glove, they can be compared with broken glass from the crime scene by putting both in a bath of silicone oil. The refractive index of the oil - how the light bends as it passes through it - changes as it is heated. When the refractive index of the glass is the same as that of the oil, the glass seems to disappear. If the pieces of glass disappear at different moments, they could not have come from the same window.

Cars have many coats of paint on them. It's possible to see the layers, and their colours, under the microscope. Paint from a suspect car can be compared with paint found at the scene of an accident. If there's a match, then they could have come from the same make of car - maybe the same car.

Blood stains can be tested for blood-group. If a blood sample from the suspect is in the same group as blood found at the crime scene, then DNA profiling can follow. This compares the DNA of the two samples of blood. A match would mean that they almost certainly came from the same person.

Forensic science can help the police to arrest and charge the right person, and can help the law courts to ensure that the right person is convicted.

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