Trace Evidence Examination Questions and Answers

Answer: B

When the temperature of a liquid is raised its refractive index decreases. A hotter liquid is less dense, so light slows down less in it. The effect is used in forensic glass analysis, where a glass fragment is immersed in oil and heated until its edge disappears.

The index does not increase or stay unchanged, and it does not 'disappear', since it is a property of every transparent medium. Because the liquid's index drops as it warms, an examiner can match a glass fragment to a reference by finding the temperature at which the fragment becomes invisible in the oil, which is the basis of the method.

Answer: A

Scales are found on wool fibre. The surface of a wool fibre is covered with overlapping cuticle scales, which are used to identify it under the microscope.

Cotton is a flat twisted ribbon with convolutions, linen (flax) shows nodes and cross markings, and silk is a smooth, structureless filament. None of these has scales. A medico-legal examiner uses this feature in the study of trace fibres, since the scale pattern of an animal hair or fibre can be compared with a reference sample under a microscope, whereas the plant fibres and silk are identified by other features such as the lumen, twist and cross-section.

Answer: B

The scales of hair can be preserved with the help of nail polish. A thin layer of clear nail polish is spread on a slide, the hair is pressed on to it, and when it dries and the hair is removed, the cuticle scale pattern remains as a cast that can be examined under the microscope. Wax, plaster of Paris and plasticine are used to take casts of tool marks, footprints and similar large impressions, and are too coarse to record the fine scale pattern of a hair. So B is correct.

Answer: B

Pyrolysis gas chromatography is useful in characterising the binder of a paint, which is the organic polymer that holds the pigment, because heating the film breaks the polymer into fragments that give a pattern that can be compared. The pigments are mostly inorganic and are studied by other methods, and the solvent evaporates from a dry paint, so neither suits pyrolysis. Lacquer is a type of paint and not one of its constituents. So B is correct.

Answer: B

Radial fractures on glass start on the surface opposite the side of impact. When a pane is struck, it bends, and the face away from the blow is stretched and fails first, so the radial cracks begin there and run outward, while the concentric cracks begin on the side of impact. So B is correct, and the edge and the corners of the glass are not where the radial fractures start. The cracks seen in broken glass are of two kinds, radial and concentric. The radial cracks run outward from the point of impact like the spokes of a wheel and begin on the side opposite the blow, where the glass is stretched, and the concentric cracks form a ring around the point and begin on the side that was struck. The examiner uses this to say from which side the glass was broken.

Answer: A

Human hair under the microscope shows a thick cortex and a thin medulla. The medulla is thin and often absent or fragmented, and the medullary index, the ratio of the diameter of the medulla to that of the hair, is generally below one third.

A thin cortex with a thick medulla is typical of many animal hairs, such as those of a rabbit or cat, which have a wide medulla. Equal thickness of cortex and medulla is also not typical of human hair. So option A is correct and None of the above is not.

Answer: D

The correct matching is vehicle with pigment distribution, lacquer with fast drying coatings, varnish with a solution of drying oils and resins in organic solvents, and plasticizer with increasing the flexibility of a polymer. This is code D, 2 3 4 1. The vehicle is the liquid in which pigment is carried and spread. A lacquer dries quickly by evaporation of its solvent.

Options A, B and C give the plasticizer or the vehicle a role that belongs to another term, for example making the vehicle the plasticizer, so they are wrong.

Answer: C

The correct matching is tempered glass with dice formation, borosilicate glass with the constituent boron trioxide, coloured glass with metallic oxides, and rock wool with molten rock. This is code C, 3 4 1 2. Tempered glass breaks into small blunt pieces like dice. Borosilicate contains \(B_{2}O_{3}\). Metal oxides colour glass. Rock wool is spun from molten rock.

Option A, B and D give tempered glass molten rock or metallic oxides, which is wrong because they do not describe the way it breaks.

Answer: B

Rayon is a regenerated fibre. It is made from cellulose of wood pulp that is dissolved and then spun and regenerated as filaments, so it is regenerated cellulose.

Cotton and silk are natural fibres, from the cotton plant and from the silk worm. Nylon is a synthetic polyamide made from chemicals. Only rayon is made by regenerating a natural polymer.

Answer: C

The annealing temperature for ordinary glass is 500 to 650 degrees Celsius. Glass is held at this temperature, below its softening point, so that internal stresses are relieved, and then cooled slowly.

200 to 350 and 100 to 250 degrees are too low for the stresses to relax. 800 to 950 degrees is near the softening and working range of soda lime glass, where it would deform. So the third option is the right range.