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Microscopy – A brief Introduction

Microscopy is field of science which deals with the use of microscope to visualize the objects that are invisible with unaided eyes. Microscope is the instrument that is used to view objects which are too small to be visible with naked eyes. The term “microscopic” was given referring to its meaning that ‘invisible to the eye unless a microscope is used’. The term “microscope” is taken from two Greek words; “micros” (meaning small) and “skopein” (meaning to see or look).

How Microscopy Works?

The view objects are separate and distinct identities are based on two properties –

  • Degree of contrast between the objects
  • Resolving power (resolution) of the view

A human eye is not capable of resolving when the distance between two distinct points is less than 0.1 mm. So microscope does not make magnification alone, it alters resolution also.

Categories of Microscopes –

Microscopes are categorized into the two different types based on their magnification.

  • Optical or light microscope – a system of optical lenses is used to alter the light waves to magnify the objects. These include –
    • Bright field microscopy
    • Dark field microscopy
    • Fluorescence microscopy
    • Phase contrast microscopy
    • Interference microscopy
  • Electron microscope – in this type of microscopy, beams of electrons are used for imaging. These include –
    • Scanning electron microscope (SEM)
    • Transmission electron microscope (TEM)

Resolving power of a microscope –

The minimum distance between two points which is visible through a microscope (lenses) is termed as resolution of a microscope. The ability of a microscope to distinguish between two separate objects is called the resolving power of the microscope. Sometimes, blurred images may be seen through the lenses. This is because of the placing of two distinct points too close to each other, which results in overlapping of the images. Magnification can neither improve nor decrease the resolving power of the microscope. The resolution can be calculated by the Abbe’s formula given below –

Resolution (d) = [0.61 λ] / [n Sin α] (in µm)]

Where,

  • λ – wave length of the light used (in µm)
  • n – Refractive index of the medium (liquid or air)
  • α – ½ of the angle of aperture
  • 0.61 is a constant
  • n Sin α – numerical aperture of the microscope

Facts to know about the microscope –

  • When the numerical aperture (NA) is higher, then it is considered as a good microscope.
  • The smaller is the value of‘d’, the better will be resolution. Lowering the value of λ and increasing the value of NA will lower the value of‘d’.

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