A microscope may be defined as an optical instrument consisting of a lens or a combination of lenses for making enlarged or magnified images of minute objects. Depending on the number of lenses, microscopes are classified into 2 types: Simple and Compound microscopes. Depending on the number of eyepieces there are 2 types: monocular (single eyepiece) and binocular (two eyepieces) microscope.
Microscopes are classified as light and electron microscopes depending on the source of illumination.
The compound microscope consist of 3 major systems:
- Support system: It comprises of base, stage and body.
- Illumination system: It throws light on the object for proper viewing. It comprises of light source or mirror, iris diaphragm and condenser. The light source may be concave or plain mirror or electrically illuminated by tungsten filament lamp or a halogen lamp, mirror and electric light source are generally interchangeable.
- Magnification system: it includes a set of lenses aligned in such a manner on that magnified real image can be viewed. The objective is a set of lenses placed near the object. It practically magnifies the object which can be observed through eyepiece in more magnified form.
PARTS OF COMPOUND MICROSCOPE:
- Ocular: A series of lenses (5x, 6x, 10x, 16x) that magnify the object and corrects some of the defects of the objectives. Huygenian, Ramsden and Compensating oculars are commonly used in microscope.
- Objectives: The objective is the most important lens of microscope because its properties make final image. The objective lenses generally equipped with microscope are low power, high power and immersion lens having magnification of 10x, 40x or 45x and 100x. Functions of objective lens are to gather the light at point of image and magnify the image. There are 3 types of objective like achromatic, fluorite and apochromatic.
- Condenser: This component is found directly under stage and contains two set of lenses that called and concentrate light passing upward from the light source into lens system. There are several different types of condensers depending upon type of microscopy. Eg: Abbes condenser, variable, focus condenser and achromatic condenser.
- Iris diaphragm: It is equipped with condensers. It controls intensity of light entering the condenser. The lever is equipped with it to adjust light intensity.
- Illumination (light source): The light source is positioned in base of instrument. Some microscopes are equipped with a built in light source to produce direct illumination. Others are provided with mirror with one side flat and other concave. An external light source such as lamp is placed in front of mirror to direct the light upward into lens system. The flat side of mirror is used for artificial light and concave side for sunlight.
- Body tube: Above the stage and attached to arm of microscope is body tube. The upper end of the tube contains ocular or eyepiece lens. The lower portion consist of movable nose piece containing objective lens. It also provides sufficient space for image formation.
- Resolving nose piece: A base in which objective are fixed and it hold 2 to 4 objectives and which can be revolved to align required objectives.
- Focus adjustment knobs: There are two focus adjustment knobs, a coarse adjustment and a time adjustment. Corse adjustment knob is used to bring abject into focus and a fine adjustment knob is used for fine and clear focus of specimen.
- Mechanical stage: It is a platform on which the glass slides is placed to view specimen. Some stages have slips to hold glass slides. Others have a mechanical stage which makes it possible to move slide across the stage
The numerical aperture may be defined simply as the diameter of its length to its focal length. And it is expressed as NA = n Sin θ
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