1. Unit – 2
INTRODUCTION TO
FORENSIC SCIENCE

2. Laws and Principles of Forensic Science
• Forensic science is concerned with the reconstruction of unique
events — attempting to determine
– what happened
– how it happened
– where and when it happened
– who was involved
• Each crime occurs under a different combination of circumstances
affected by an enormous number of variables, and is thus
nonreplicable
• Forensic scientists deals with samples that are limited in size and
quality, and which have an unknown history
• Legal process imposes unique constraints and characteristics on
forensic science

3.  Law of individuality
 Principle of exchange
 Law of progressive change
 Principle of comparison
 Principle of analysis
 Law of probability
The laws and principles of all the natural sciences are
the bases of forensic science. In addition, it has
developed its own principles which are as follows:

4. • Every object, natural or man-made, has an individuality which is not
duplicated in any other object. This principle, at first sight appears to be
contrary to common beliefs and observations.
• The grains of sand or common salt , seeds of plants or twins look exactly
alike. Likewise, man-made objects: coins of the same denomination
made in the same mint, currency notes printed with the same printing
blocks one after the other (excluding serial number) and typewriters of
the same make, model and batch appear to be indistinguishable. Yet the
individuality is always there. It is due to small flaws in the materials, in
the arrangement of the crystals, imperfect stamping or due to inclusions
of some extraneous matter. The individuality has been verified in certain
fields.
• The law of individuality is of fundamental importance in forensic
science. Anything and everything involved in a crime, has an
individuality. If the sane is established , it connects the crime and the
criminal.
Law of individuality

5. • Contact exchanges traces is the principle of exchange. It was first
enunciated by the French scientist, Edmond Locard.
• According to the principal, when a criminal or his instruments of crime
come in contact with the victim or the objects surrounding him, they
leave traces. Likewise, the criminal or his instruments pick up traces from
the same contact. Thus, a mutual exchange of traces like takes place
between the criminal, the victim and the objects involved in the crime If
these traces are identified to the original source, viz., the criminal or his
instrument (or vice versa), they establish the contact and pin the crime on
to the criminal. The principal of exchange is aptly demonstrated in hit and
run cases and in offences against person.
If the investigating officer is able to establish the points of contact , he is
likely to gather physical clues:
• If a criminal enters the premises through a ventilator , he leaves his
foot prints in dust on the sill.
• If he breaks a window or a door, the jimmy leaves its marks on the
wooden frame.
Principle of exchange

6. Principle of recovery
• Do not harm (evidence)
• Nothing should be added, damaged or obliterated in the recovery
process
• Particular attention should be paid to avoiding contamination, a
concern that gains increasing importance with each advance in
analytical sensitivity.
• Where there is risk of losing or damaging evidence, appropriate experts
should be called in.
• Exhibit items should be safely and securely packaged as soon as
possible.
• If an object is portable, transfer to the lab for evidence recovery under
controlled conditions.
• Do no harm to yourself! Crime scenes and recovered evidence may
pose biological or chemical hazards.
• Health & safety measures for collecting and transporting evidence.

7. Everything changes with the passage of time. The rate of change varies
tremendously with different objects.
1. The criminal undergoes rapid changes. If he is not apprehended in time, he
becomes unrecognizable except perhaps through his fingerprints, bone
fractures or other characteristics of permanent nature which are not always
available.
2. The scene of occurrence undergoes rapid changes. The weather, the
vegetable growth, and the living beings (especially human-beings) make
extensive changes in comparatively short periods. Longer the delay in
examining the scene, greater will be the changes. After some time ,the
scene may become unrecognizable.
3. The objects involved in crime change gradually, the firearm barrels loosen ,
metal objects rust, the shoes suffer additional wear and tear and the tools
acquire new surface patterns. In course of time the objects may loose all
practical identity vis-à-vis a particular crime.
The principle, therefore, demands prompt action in all aspects of criminal
investigation.
Law of progressive change

8. • Improper sampling and contamination render the best analysis
useless. The principle emphasizes the necessity of correct
sampling and correct packing for effective use of experts.
For example:-
A small amount of dust is recovered from a small sticky patch of
the shoe of a culprit. The investigating officer collects about two
kilograms of soil from the scene packs it in tin and sends it as a
control sample. The results of comparison are inconclusive.
Why?
Principle of analysis

9. Only the like can be compared is the principle of comparison.
It emphasizes the necessity of providing like samples and specimens
for comparison with the questioned items:
1. In a murder case, a bullet is recovered from the deceased. The
expert opines that the bullet has been fired from a firearm firing
high velocity projectiles like a service rifle. It is futile to send
shotguns, pistols or revolvers as the possible suspect firearm.
2. A bunch of hair is recovered from the hands of a deceased. The
expert opines that the hair belong to a Negroid person. Hair from
persons of white races for comparison will not be of any use.
3. The questioned writing is found to have been writing with a ball pen.
To send fountain pen as a likely instrument of writing is futile.
Principle of comparison

10. • All identifications, definite or indefinite, are made, consciously
or unconsciously, on the basis of probability.
• Probability is the mathematical concept.
• It determines the chances of occurrence of a particular event in
a particular way out of a number of ways in which the event can
take place or fail to take place with equal facility.
If P= represents probability
Ns = the number of ways in which the event can successfully
occur (with equal facility) and
Nf = the number of ways in which it can fail(with equal facility),
Then the probability of success is given by the formula:
P= Ns/ Ns+ N2.
Law of Probability

11. Principle of Presentation
• Working within an ethical framework, a forensic scientist should
fully disclose and present impartial evidence which is readily
understandable and neither overstated nor understated.
• Full disclosure of all facts, assumptions, data, conclusions and
interpretations should be made.
– Lab report: Complete & retained.
• He or she should not be an advocate for either side of the case
at hand; it is, however, to be expected that a forensic scientist
will be an advocate for his or her opinion.
• However, the forensic scientist should be prepared to change an
opinion whenever background circumstances or assumptions
change, or new information becomes available.

12. Branches of Forensic Science

13. Toxicology
• Toxicology deals with the detection of toxic substances and
drugs in body tissues and fluids.
• The toxicologist analyzes biological fluids and tissues from
victims who are thought to have been poisoned accidentally or
purposely.
• The toxicologist, as distinct from the forensic chemist,
primarily handles biological materials and can detect poisons
in blood, urine, spinal fluid, gastric contents, bile, and tissues.

14. Anthropology
• Forensic anthropologists are experts in the identification of
bones and skeletal remains. Their studies provide information
about sex, race, age, and time of death.
• They may also lend support to investigations concerning living
cases such as a mix-up of children in a hospital nursery or
skeletal identification of persons involved in immigration
problems.
• The forensic anthropologist may be extremely helpful in mass
disasters with considerable skeletal remains or in cases of mass
burials.
• A recent application of this expertise was reconstructing the face
of a skeletonized head.

15. Odontology
• The odontologist, or dentist, provides information through
examination of teeth and dental prostheses. Victims of a disaster
or homicide may be identified by a comparison of their dental
charts and X-rays to the dental evidence from the victims.
• Bitemarks in apples, cheese, chewing gum, and other media, as
well as on a victim’s body, may be studied by these scientists.
• This form of expertise may also be used in living cases, as the
examination of teeth is helpful where there is a possibility of a
hospital mix-up of children, as such examinations may depend
upon the presence on inherited characteristics in teeth.

16. Biology
• The examination of plant life, insects, soil, trees, dirt, seeds,
and pollen, as well as blood analysis, can be a means of
developing new resources to a forensic investigation

17. Questioned Documents
• Questioned document (QD) examinations were reported as far
back as Roman times, when cases of forged documents were
described.
• The modern use of typewriters and computer printers has added
significantly to the activities of questioned document examiners.
• The QD examiner’s work includes the examination of
handwriting, ink, paper, typewriter or printer impressions, or any
other form of writing or printing that may have been used in a
case.
• This expertise includes detection of counterfeiting and various
types of fraud involving government paper, checks, forms,
money, and credit cards or the possible falsification of entries in a
ship’s log.
• The investigation of computer fraud may also examine the
validity of printout material

18. Forensic psychology
• It is the intersection between psychology and the justice system.
It involves understanding criminal law in the
relevant jurisdictions in order to be able to interact appropriately
with judges, attorneys and other legal professionals.
• It is concerned with collecting, examining, and presenting
evidence to help facilitate a legal decision. Within the legal
system, forensic psychology provides a means for being able to
enforce the law while taking human behavior into consideration.
• A forensic psychologist may be called upon to differentiate
between criminal behavior and human behavior that may be
caused by a psychological imbalance.

19. Physics
• Types of evidences examined
The application of physics for purposes of civil or criminal law.
• Indirectly, physics has contributed to forensic science via the
invention of the microscope, the electron microscope, the mass
spectrometer, and optical spectrometers; but directly, its role in
forensic science has been minimal.
• Forensic physics has traditionally involved the measurement of
density (soil and glass examination), index of refraction, and
birefringence (fiber analysis, glass examination).

20. Ballistics
• Ballistics is the science of mechanics that deals with the flight,
behavior, and effects of projectiles, especially bullets, gravity
bombs, rockets, or the like; the science or art of designing and
accelerating projectiles so as to achieve a desired performance.

21. Forensic Entomology
• Forensic entomology is the study of the insects associated with a
human corpse in an effort to determine elapsed time since
death.
• Insect evidence may also show that the body has been moved to
a second site after death, or that the body has been disturbed at
some time, either by animals, or by the killer returning to the
scene of the crime. However, the primary purpose of forensic
entomology today is to determine elapsed time since death.

22. Judicial officer
• A judicial officer is a person with the responsibilities and powers
to facilitate, arbitrate, preside over, and make decisions and
directions in regard to the application of the law.
• Judicial officers are typically categorized
as judges, magistrates, puisne judicial officers such as justices of
the peace or officers of courts of limited jurisdiction;
and notaries public and commissioners of oaths.
• The powers of judicial officers vary and are usually limited to a
certain jurisdiction.

23. Role of Police
• Securing the scene
• Questioning
• Assisting in evidence collection
• Documenting
• Arresting/taking into custody
Role of a medico legal expert
• Necessary to determine the cause of death.

24. Forensic Science Lab
– Processing of the scene of crime, collection, handling,
preservation, packaging of the physical evidence
– Sample collection
– Quick spot-test kits
– Scanning devices: UV, IR scanning devices
– Items: Gloves, Bags, cotton, cardboard boxes, labeling tools
– Photography

25. 1. Observe a problem or questioned evidence.
2. Consider a hypothesis to know the way of crime.
3. Examine, test and analyze those evidence.
4. Determine the significance of the evidence.
5. Formulate a theory based on evaluation of the significance of
the evidence
Forensic scientist has to