MAJOR OPTICAL PARTS OF A MICROSCOPE

Microscopic examinations of bacteria culture, Permag Laboratory Port Har-court

Before we discuss the major optical parts of a microscope, let us first of all examine what it is. 

WHAT IS A MICROSCOPE?

This is simply an instrument where lenses are arranged in such a way that an object is magnified several times before the final magnification in the observer’s eye lens.  It increases the size of an object up to and beyond 1.0nm, the threshold size value below which the human eyes can see an object.

 Magnification is the artificial increase in size and dimension of an object. Larger magnifications are usually produced by setting the condenser to produce Koehler Illumination which uses parallel rays of light as illuminator.  

There are different types of microscope, we have the

1.       Bright field microscope

2.       Interference  Contrast Microscope ( ICM)

3.       Transmission Electron Microscope (TEM)

4.       Scanning electron Microscope

5.       The scanning tunneling Microscope( STM)

6.       Fluorescence Microscope

7.       Immunofluorescence Microscope (IM)

8.       Dark  Field Microscope

9.       Phase contrast microscope

The effectiveness of an optical instrument is limited by the property of light used in illumination, because of the wave nature of light, a very tiny object appear as a disc, surrounded by a dark and light rings, two adjacent points can be distinguished or resolved only if the rings surrounding them do not overlap.  This phenomena is called Resolving Limit, and its defined as distance between two points that can be distinguished (separated).

OPTICAL PARTS OF A MICROSCOPE

          1. Objective lens:- The objective lenses are tubes of various magnification power that are  rotated in other to use, some microscope has three while others has an additional objective known as Oil immersion  lens.

The magnification power is engraved on each objective, and they produce clear images. 

         a. 4X or 4mm objective:-This is the shortest objective, it is a yellow lens and has a magnification power of (four times) meaning four times the actual diameter of a specimen, the 4X objective is usually called the scanning lens because of it’s lower magnification that gives an overall view of the specimen.

When not in use, the 4x objective should be place pointing down to reduce accidental ramming of the objective into a microscope slide during focusing.

          b- 10X or 16mm objective: - This is the low power objective and it is a blue lens.

          c- 43X or 45mm:- This is the high power objective.

          d- 90x or 100mm:- this is the oil immersion lens, as the name implies, it requires oil for use, usually a drop is place on the slide. It is the white lens.

Immersion oil increases the resolution power of the lens because it has the same refractive index as glass.

          2. Eye piece:- these are also known as ocular lenses and can be monocular or binocular.  Ocular lens further magnifies an image and deliver it to the eyes or camera.  The degree of enlargement of an object I its magnification.

Eye piece increase magnification but does not reveal more details, this is known as empty magnification.

Empty magnification is one that does not increase detail of object observed while useful magnification increases fine detail and it is controlled by the wavelength of light use.

           3. Condenser: This is a mirror that focuses light ray on a specimen, the microscope is place in a position where it can receive enough day light and the condenser used to concentrate light rays on the stage to illuminate the specimen on the specimen. The condenser is used especially in the absence of electricity.

           4. Irish Diaphram: This is an aperture in form of a ring that can be adjusted to control the amount of light reaching a specimen.

The amount of light is important for controlling the contrast, resolution and depth of field.

Resolution and contrast are antagonistic as improving one will lead to the loss of the other.

Resolution is increased by increasing the amount of light; however, brighter light leads to loss of contrast.

Contrast is the amount of light reaching a specimen while depth of field is the thickness of the specimen that will be in acceptable focus.

What is resolution?

Resolution is the ability of lens to separate or distinguish between small objects that are close apart, in other words, it is the maximum distance at which two distinct points can be seen as separate entities.

 Resolution is explained sing German physicist Enerst Abbe’s (1870’s) equation.

           d             =      0.5λ

                          N Sine a

           λ             =  The wavelength of the light source employed

           a             =  The half angle of the object lens.

         N         = The refractive index of the medium between the specimen and the front of the object lens.

Note also that N Sine a =Numerical aperture (NA).

(NA), this is the light gathering power of the objective. Up to a certain limit, increasing NA of an objective lens increases its resolving power. However, when the medium between the specimen and the objective is air, a feasible diameter of the objective lens limits the NA to approximately 0.65.

But the refractive index (N) can be increased by using oil as the intervening medium between the specimen and the objective lens since oil has higher N than air. With oil, the NA can be increased to a value as high as 1.4 (1.25 being more common).

Under optimum working condition, the maximum resolution of the light microscope approaches 200mm, with light of the shortest visible wavelength (approximately 426nm). The implication is that two adjacent points closer together than 200nm cannot b e resolved into separate images using light microscope.