Introduction
to Microscope
Microscopy
is the science of using microscopes to observe objects that are too small to be
seen with the naked eye. It plays a vital role in a wide array of fields
including biology, medicine, materials science, and chemistry.
The
Microscope is an optical instrument consisting of a combination of lenses for
making enlarged or magnified images of minute objects. The term is derived from
the two Greek words ‘micro’ - small, and ‘scope’ - view. Since microorganisms are very small and
invisible to naked eyes, they must be magnified to be clearly seen. The use of
a microscope is absolutely indispensable to study microbiology.
Working
of lenses
When
a ray of light passes from one medium or material to another, refraction occurs,
the light ray will bend at the interface.
Refractive index is the measure of how a medium or material slows the
velocity of light. Glass has higher
refractive index and air have lower refractive index The direction and
magnitude of bending is determined by the refractive indexes of the two media or
materials forming the interface. When light passes from air into glass, it is
slowed and bent toward the normal, a line perpendicular to the surface and as
light leaves glass and returns to air, it accelerates and is bent away from the
normal. As shown in the below image, a
prism bends light because of the different refractive indices of glass and
air.
The Bending of Light by a Prism
(Prescott−Harley−Klein:Microbiology, Fifth Edition)
Lens
act like a collection of prisms operating as a unit. When parallel rays of
light strike the lens, a convex lens will focus these rays at a specific point
termed as the focal point (F). The distance between the centre of the lens and
the focal point is called the focal length (f).
Lens strength is related to focal length; a lens with a short focal length
will magnify an object more than a weaker lens having a longer focal length.
Lens functions like a collection of prisms
(Prescott−Harley−Klein:Microbiology,
Fifth Edition)
Magnification
and resolving power
Magnification
and resolving power are important concepts in microscopy and imaging. Magnification is the process of making an
object appear larger than its actual size. Magnification is expressed as the
ratio of the object's apparent size to its actual size. Resolving power or Resolution is the ability
to distinguish two objects that are close together. While higher magnification
can increase the amount of detail in an image, it doesn't necessarily improve
the ability to distinguish fine details. For example, two images may have the
same magnification, but one may have a higher resolution and be clearer.
The
minimum distance (d) between two objects that reveals them as separate entities
is given by the Abbe equation, in which lambda (λ) is the wavelength of light
used to illuminate the specimen and n sin θ is the numerical aperture (NA).
Numerical
aperture
The
measure for the resolving powers of a lens is the numerical aperture. It is n sin θ. The larger the numerical aperture, then the
greater is the resolving power of the lens. Theta is defined as 1/2 the angle
of the cone of light entering an objective. Light that strikes the microorganism
after passing through a condenser is cone- shaped. When this cone has a narrow
angle and tapers to a sharp point, it does not adequately separate images of
closely packed objects and thus the resolution will be low. If the cone of
light has a very wide angle and spreads out after passing through a specimen, closely
packed objects appear widely separated and thus the resolution will be high.
The
angle of the cone of light that enter a lens depends on the refractive index (n)
of the medium in which the lens works. The refractive index for air is 1.00.
Since sin θ cannot be greater than 1 (the maximum θ is 90° and sin 90° is
1.00), a lens working in air cannot have a numerical aperture greater than
1.00. In order to raise the numerical aperture above 1.00, to achieve higher
resolution, the refractive index is to be increased. This could be achieved by
using immersion oil, a colorless liquid having the same refractive index as
glass (about 1.515). If air is replaced with immersion oil, by adding a drop of
oil on the surface of glass slide containing the specimen, light rays that otherwise
may not enter the objective due to reflection and refraction will now enter the
lens. So, an increase in numerical aperture and thus
increase in resolution results.
Oil
immersion objective operating in air and with immersion oil (Prescott−Harley−Klein:Microbiology,
Fifth Edition)
Types of Microscopes
A
simple microscope consists merely of a single lens or magnifying glass held in
a frame, usually adjustable, and will have a stand for holding the object to be
viewed and a mirror for reflecting light. A compound microscope consists of two
sets of lenses, one known as an objective and the other as an eyepiece. Compound microscopes give more magnification
than simple microscope.
There
are different types of Microscopes. The major being a) Light Microscopes
(Optical Microscopes) and b) Electron Microscopes
A.
Light Microscope (Optical Microscope):
Light
microscopes use visible light and lenses to magnify objects. Magnification typically ranges from 40x to
1000x.
Compound
microscope and its parts (Fundamental Principles of Bacteriology : A.J. Salle)
Parts
of a Light Microscope:
Eyepiece
(ocular lens): The lens you look through, typically 10x magnification.
Objective
Lenses: These are multiple lenses that provide different magnifications
(usually 4x, 10x, 40x, 100x).
Stage:
The flat surface where the slide with the specimen is placed.
Condenser:
Focuses the light onto the specimen.
Diaphragm:
Controls the amount of light that passes through the sample.
Coarse
and Fine Focus Knobs: Adjust the focus of the image.
Illuminator:
A light source, usually a lamp, that illuminates the sample.
Types
of Light Microscopy:
Bright-Field
Microscopy: The most commonly used technique where light passes through the
specimen, and the image appears dark against a bright background.
Dark-Field
Microscopy: Light is directed at an angle to the sample, which makes the image
appear bright against a dark background.
Phase-Contrast
Microscopy: Enhances contrasts in transparent specimens and is useful for
studying live cells.
Fluorescence
Microscopy: Uses ultraviolet (UV) light to excite fluorophores attached to
specimen. When the fluorophores emit light, the specimen appears as brightly coloured
on a dark background.
B)
Electron Microscopes (EM)
Electron
microscopes use electrons instead of light to view specimens at much higher
magnifications and resolutions than light microscopes.
Transmission
Electron Microscope (TEM):
TEM
transmits a beam of electrons through a thin sample. The electrons interact
with the sample, and the resulting image is projected onto a screen or film and
can achieve magnifications up to 10 million times and capable of Resolution at
nanometer scale (less than 1 nm).
Scanning
Electron Microscope (SEM):
SEM
scans a focused electron beam over the surface of the sample. The electrons
interact with the surface and produce signals that are used to form an image. Can
achieve magnifications up to 1 million times and resolution of about 1-10
nanometers.
C.
Scanning Probe Microscopes (SPM)
Scanning
Probe Microscopes work by scanning a sharp probe across the surface of the
specimen. The most common type is the Atomic Force Microscope (AFM). This measures the force between the probe and
the surface and help for the construction of a high-resolution topographical
image.
D.
Other Advanced Microscopy Techniques
Confocal
Microscopy uses laser light to scan specimens and produce high-resolution 3D
images. Confocal microscopes capture optical sections of the sample, which can
then be digitally reconstructed to create a 3D image.
Multiphoton
Microscopy is a type of fluorescence microscopy that uses multiple photons to
excite fluorescent dyes.
Live-Cell
Microscopy is used to observe living cells in real-time, often with the help of
fluorescent dyes or proteins.
Super-Resolution
Microscopy includes techniques like STED (Stimulated Emission Depletion) and PALM
(Photo-Activated Localization Microscopy).
These provide resolutions of less than 100 nm.
The
Bright-Field Microscope
The
ordinary microscope is called a bright-field microscope because it forms a dark
image against a brighter background.
The
microscope consists of a sturdy metal body or stand composed of a base and an
arm to which the remaining parts are attached.
A
light source, either a mirror or an electric illuminator, is located in the
base.
Two
focusing knobs, the fine and coarse adjustment knobs, are located on the arm
and can move either the stage or the nosepiece to focus the image.
The
stage is positioned about halfway up the arm and holds microscope slides by
either simple slide clips or a mechanical stage clip. A mechanical stage allows
the operator to move a slide around smoothly during viewing by use of stage
control knobs.
The
substage condenser is mounted within or beneath the stage and focuses a cone of
light on the slide. Its position often is fixed in simpler microscopes but can
be adjusted vertically in advanced models.
The
curved upper part of the arm holds the body assembly, to which a nosepiece and
one or more eyepieces or oculars are attached. More advanced microscopes have
eyepieces for both eyes and are called binocular microscopes.
The
nosepiece holds three to five objectives with lenses of differing magnifying
power and can be rotated to position any objective beneath the body assembly.
Ideally a microscope should be parfocal—that is, the image should remain in
focus when objectives are changed.
The
objective lens forms an enlarged real image within the microscope, and the
eyepiece lens further magnifies this primary image. The total magnification is
calculated by multiplying the objective and eyepiece magnifications together.
For example, if a 45X objective is used with a 10X eyepiece, the overall
magnification of the specimen will be 450X.
The resolution of a microscope depends upon the numerical aperture of its condenser as well as that of the objective.
The resolution of a light microscope can be calculated using the Abbe equation. The maximum theoretical resolving power of a microscope with an oil immersion objective (numerical aperture of 1.25) and blue-green light is approximately 0.2 µm.
So,
a bright-field microscope can distinguish between two dots which are 0.2 µm
apart, the size of a very small bacterium.
