Name: Sneha Shinde
Course: Intg. MSc+PhD Biotechnology
Roll No.: MSB/PH/2022/001
Enrollment No.: A005159222001
Dr. Manoranjan Nayak
Microorganisms are too small to be seen by our unaided eyes and the
microscopes are of crucial importance as they help to view the microbes. A
microscope is an optical instrument consisting of one or more lenses in order
to magnify images of minute objects. Thus it is important to gain a preliminary
knowledge about the principles of microscope and its types.
Microscope, as evident from its name, is a device that enables us to see
objects that are invisible to the naked eye. The name literally roughly
translates to seeing small objects, where Micro means ‘small’ and scope
means ‘to see’ (derived from Ancient Greek).
Antony Van Leeuwenhoek invented the first simple microscope in 1683. In
simple microscope, single lens is used for magnification whereas in compound
microscope, multiple lenses are used. Monocular microscope has one
eyepiece and binocular has two eyepieces. As research has advanced, we now
have different types of microscopes that are commercially available.
The microscope works on three principles of physics:
2. Resolving power
3. Numerical aperture
Magnification: It is the ability of lenses to enlarge an object visually. If the magnifying power of lens is 10X it
means that the given lens can enlarge the object up to 10 times. In compound microscope, the magnification is
the product of magnifying power of both the lenses. The magnifying power of lens depends on focal length. Lower
the focal length, higher is the magnifying power of lens.
Resolution: The resolving power is the ability to distinguish two closely placed points. The resolution power of lens
allows us to observe the details of an object. The resolution of microscope can find out by using Abbe’s equation.
d – distance between two closely distant points
λ – wavelength of light
n sin θ – numerical aperture
The microscope with higher magnification has small d value. λ is the wavelength of light, shorter is the wavelength;
higher is the resolution. The wavelength of visible light is from 300 to 700 nm. The best resolution for light
microscope is obtained in the range of 450 to 500 nm. ‘n’ is the refractive index of medium.
Refractive index is the ability of the medium to bend the light. The angle of cone of light is affected by the
refractive index of medium. The refractive index of air is 1. ‘θ’ is the half of the angle of the cone of light that
enters the microscope. The value of ‘Sin θ’ cannot be more than 1 because angle of entering cone of light cannot
be more than 90° and value of sin 90 is 1. d= 0.5 x 450 nm 1 d= 225 nm 0r 0.2 μm Hence, the resolution limit of
light microscope is 0.2 μm.
Important parts of the microscope:
Base: It gives support to the body. The illuminating source of microscope is placed at the
base of microscope
Arm: It is used to hold and carry the microscope. The coarse and fine focusing knobs are
placed on the arm.
Eyepiece: It is the part through which we observe the sample/object. The magnifying
power of the eyepiece lens is generally 10X.
Eyepiece tube or body tube: The function of eyepiece tube is to hold the eyepiece and
hence it is names as eyepiece tube.
Nosepiece: The flexibility of nosepiece allows switching the objective lenses.
Objective lens: In general 3 objective lenses are placed. Their magnifying power is 10X,
40X and 100X respectively.
Focusing mechanism (Adjacent knobs): It is used for right placing of sample and for
Stage: It is place where sample or object is kept for viewing. For accurate placement, it is
provided with clips that hold the slide firmly.
Aperture: It is present on the stage that allows the entry of light from illuminator to fall
on sample or object.
Source of Illumination: In non-electric microscope, the sunlight is used as source of
illumination and hence mirror is placed to focus the sunlight. In electric microscope, lamp
is placed of specific wavelength.
Condenser: Its function is to focus the light on sample form illuminator.
Diaphragm: It is generally associated with condenser. The diaphragm and condenser
together produce hollow cone of light that strikes the sample and illuminate it.
Types of microscope:
Light Microscope: Magnification is obtained by a system of optical lenses using light waves.
(i) Bright field
(ii) Dark field
(iv) Phase contrast and
(v) UV Microscope
Electron Microscope: A system of electromagnetic lenses and a short beam of electrons are used to
(I) Transmission electron microscope (TEM)
(II) Scanning electron microscope (SEM)
This instrument contains two lens systems for magnifying specimens: the
ocular lens in the eyepiece and the objective lens located in the nose-piece.
The specimen is illuminated by a beam of tungsten light focused on it by a sub-
stage lens called a condenser, and the result is that the specimen appears dark
against a bright background. A major limitation of this system is the absence of
contrast between the specimen and the surrounding medium, which makes it
difficult to observe living cells. Therefore, most bright field observations are
performed on nonviable, stained preparations.
This is similar to the ordinary light microscope; however, the condenser system
is modified so that the specimen is not illuminated directly. The condenser
directs the light obliquely so that the light is deflected or scattered from the
specimen, which then appears bright against a dark background. Living
specimens may be observed more readily with dark field than with bright field
microscopy. Maximum magnification of 1500x and resolution of 0.1 – 0.2 μm
can be obtained. It is useful in studying the morphology and motility of
microorganisms. Dark field is especially useful for finding cells in suspension.
Dark field makes it easy to obtain the correct focal plane at low magnification
for small, low contrast specimens.
This microscope is used most frequently to visualize specimens that are
chemically tagged with a fluorescent dye. The source of illumination is an
ultraviolet (UV) light obtained from a high-pressure mercury lamp or hydrogen
quartz lamp. The ocular lens is fitted with a filter that permits the longer
ultraviolet wavelengths to pass, while the shorter wavelengths are blocked or
eliminated. Ultraviolet radiations are absorbed by the fluorescent label and the
energy is re-emitted in the form of a different wavelength in the visible light
range. The fluorescent dyes absorb at wavelengths between 230 and 350 nm and
emit orange, yellow, or greenish light. This microscope is used primarily for the
detection of antigen-antibody reactions.
Observation of microorganisms in an unstained state is possible with this
microscope. Its optics include special objectives and a condenser that make
visible cellular components that differ only slightly in their refractive indexes. As
light is transmitted through a specimen with a refractive index different from that
of the surrounding medium, a portion of the light is refracted (bent) due to slight
variations in density and thickness of the cellular components. The special optics
convert the difference between transmitted light and refracted rays, resulting in a
significant variation in the intensity of light and thereby producing a discernible
image of the structure under study. The image appears dark against a light
Resolution of a microscope depends upon the wavelength of light used.
If, longer the wavelength of light used, lower will be the resolving
power while shorter the wavelength, more will be the resolution. With
this principle, UV rays of shorter wavelength are used as light source.
Since UV rays can’t penetrate the glass, quartz lenses are used. Since
the UV rays are invisible, photographic plates should be used to record
the image or special type of filters should be used to eliminate the UV
rays from reaching the eyepiece. This is used in conjunction with
fluorescent microscopy. Upon illumination with UV light certain
fluorescent dyes emit light in visible range, which can be directly
Transmission electron microscope (TEM)
TEM has a projector lens that project the image onto a fluorescent viewing screen or
film plate, because the beam cannot be viewed directly. With TEM greater resolution
and higher magnifications than light and Scanning Electron Microscope can be
obtained. In TEM, the differential scattering of electrons by the specimen makes the
contrast. Since most of the atoms of the biological material are of low mass, the
contrast of the specimen is low. Staining with heavy metals such as platinum, uranium
or tungsten can increase the contrast.
They are of various types such as:
a. Positive staining: The heavy metals are fixed on the specimen.
b. Negative staining: It is used to increase the electron opacity of the surrounding
Two techniques commonly employed for the observation of biological specimen are:
a. Metal shadowing: The dried specimen is exposed at an acute angle to a stream of
heavy metals like platinum, palladium or gold and thereby producing an image,
that reveals the three-dimensional structure of the object.
b. Freeze fracturing: The frozen specimen is fractured with a knife, and the exposed
surface is coated with a heavy metal (Gold) at an acute angle. A supporting layer of
carbon is evaporated on the metal surface. Then the specimen is destroyed and
the replica is examined.
Scanning electron microscope (SEM)
The specimen is coated with a thin layer of heavy metal and the specimen is
subjected to a narrow beam of electrons, which rapidly moves and scans the
surface of the specimen. The irradiated specimen depending upon its physical
and chemical composition will release secondary electrons. These secondary
electrons are then collected by anode detector, which generates an electronic
signal. Then the electronic signal is scanned in TV system to produce an image
on a cathode ray tube. Magnification on SEM is about 75,000 to 1,00,000 times.
a. Specimen is kept under high vacuum on the path of electron beam. So, living
cells can’t be examined.
b. Electrons have low penetration capacity, hence ultra thin section and
staining should be done which is time consuming and also sometimes alter
or distort the structures of microorganisms.
c. High cost and specialized techniques prevent its use in all microbiology labs
in spite of greater magnification and resolution.