Lens System – Microscopy – Types and Information | Biology Ideas

 Some Basic about Lens

The lens is a portion of a transparent refracting medium bounded by two spherical surfaces or by one spherical surface and a plane surface.

Lenses are usually made of glass.

 

The following types of lenses are in common use, the first 3 are convergent and the last three are divergent lenses –

1. Double convex or bi-convex lens

2. Plano-convex lens

3. Concavo-convex lens

4. Double concave or bi-concave lens

5. plano-concave lens

6. Convexo-concave lens

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The compound microscope comprises three kinds of lenses

The condenser

The objective

The oculars

 

The Condenser

It is located below the stage and above the mirror or the source of light.

The function of Condenser –

To collect and focus the light rays into the plane of the object.

Condenser does not have any role in image formation.

Focusing the light increases the intensity of light. Within reasonable limits the more light the better is the resolution.

The intensity of light can be adjusted by lowering the condenser or adjusting the aperture of the diaphragm of the condenser.

Various cell components of amoeba and bacterial cell motility is observed with better contrast by lowering the condenser/ decreasing the light intensity.

Iris diaphragm –It is fitted below the condenser and can be opened to varying extent to control the intensity of light entering the condenser.

Eyepiece

Commonly used substage condensers are of three types –

Abbe

Variable focus

Achromatic

Read Also: Fundamental Microscopy | Biology Ideas | Microscopy Definition and types

 

The Objective

These are mounted on a revolving nosepiece.

Functions of the objective lens system –

To gather the light rays coming from any point of the object.

To unite the light in a point of the image

To magnify the image. It forms a real image of the object.

 

Objective in microscope

Based on magnification, three different objectives are commonly used – 

1. Low power objective – 10X

2. High power objective- 45X

3. Oil immersion objective- 100X

 

Oil Immersion Objective(O.I.O) –

Magnification is 100X

Can be used only after introducing immersion oil between the cover glass and the objective.

R.I. of oil is equivalent to glass.

When oil is a medium it prevents diffraction of light rays, resulting in a clear and sharp image at the maximum magnification.

Cover glass of thickness 0.17 mm is specifically for O.I.O.

Lenses are generally apochromats.

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Different types of objective lenses based on performance and construction –

1] Achromats

Simplest and most commonly used.

Correct aberrations mainly with regard to red and blue color.

2] Fluorites

Made of fluorite and other minerals.

Correct a wide range of aberrations especially with regard to color.

Mainly used for pathological and photographic studies.

Reveal subtle differences in the colors of the specimens.

3] Apochromats

Most highly corrected objectives and most expensive.

Made up of a variety of glass and minerals.

Correct all types of spherical and chromatic aberrations.

4] Plain Apochromats

Provide a perfectly flat field of view from edge to edge.

Eliminate aberrations caused due to curvature of the field and different focal planes from the Centre towards the edges.

Mainly suitable for photography

 

Numerical aperture [N.A.]

It determines the quality of the objective.

The power of resolution of a microscope depends entirely on the NA of the objective.

Expressed as a figure.

Engraved on the body of the objective by the manufacturer.

The formula for Numerical Aperture –

NA = n(sinϴ)

{ ‘n’– Refractive index of the medium [air, water, immersion oil] between the slide and the objective lens.

‘ϴ’ – is the semi-angle between the lens and the outermost ray which can enter the front lens.  }

 

NA value can be increased by –

1. Increase in ‘n’ value [refractive index]of the medium present between the object and the objective lens —

The Refractive index of air is 1, water is 1.33, and oil[such as cedarwood oil] is around 1.5 which is closest to the ‘n’ value of glass.

When oil is used on the specimen, the medium is homogenous with the glass of the lens and the glass slide acting as a single medium through which light has to pass.

By using O.I.O. there is minimum refraction of light, allowing maximum light coming into the specimen to pass through the objective lens.

 

2. Increase in ‘ϴ’

Light enters the object and radiates from it in all directions.

Only some rays enter into the objective lens forming an angle ‘ϴ’ with the axis.

Maximum N.A. obtained with dry objective is 0.95(for 40X objective it is 0.7), when water is used as a medium N.A. is 1.30 and with O.I.O. it is 1.4.

Higher the Numerical aperture, the better is the light gathering capacity of the lens which results in better resolving power of the microscope.

 

The Oculars

Located at the top of the body tube.

Monocular – having one ocular.

Binocular – having 2 oculars of similar magnification.

The number of oculars varies depending on the magnification required.

5X,10X,12.5X and 15X oculars are available.

Consists of several elements in each unit to avoid the various types of aberrations.

eyepiece

Functions of the oculars(eyepiece) are to –

Magnify the real image formed by the objective lens. The magnified image is virtual and inverted in the plane of the object.

Correct some of the defects of the objective.

Images crosshairs, pointers, micrometer scales, etc. located in the eyepiece.

 

Commonly used eyepieces are Huygenian (-) and Ramsden (+) eyepieces –

1] Huygenian (-)

The plane surfaces of the two lenses face upwards and the diaphragm is situated between them.

The diaphragm is at the focus of the upper (eye) lens.

The lower field lens collects light from the rays coming out of the objective and focuses them at or near the plane of the diaphragm

The eye lens then magnifies this focused image.

The diaphragm limits the field of view to the central and flattened part of the image and reduces glare.

Called ‘negative’ oculars because the focus appears within the eyepiece.

 

2] Ramsden eyepiece(Positive oculars)

It has the convex surfaces of both lenses facing inwards, the two together forming a single lens unit.

The diaphragm is placed below the lower legs.

It has the advantage that any aberration or distortion of the lenses equally affects the image itself and the view of any scale placed on the external diaphragm.

Useful for micrometry.

Give more accurate results than Huygenian eyepiece.

 

3] A pointer eyepiece

Used to pinpoint a particular structure under the microscope.

It is a simple instrument with a sharp pointer attached to an adjustable hinge-joint and a small handle.

Fixed within the ocular.

The rotatory movement of the handle is used to direct the pointer in the microscopic structures to the students.

 

4] Micrometer ocular

Used in micrometry for measurement of cell dimensions.

Has a small, transparent, glass disc on which is etched a scale.

A glass disc is placed in the body tube.

Used in combination with the micrometer stage.

 

Body tube

Located between the ocular and the nosepiece.

The standard length is 160mm.

Monocular tube- Used in student’s microscope.

Binocular tube- Comfortable viewing for prolonged periods. Has provision for adjusting the inter-pupillary distance to focus separately the image with each eye.

Combined photo binocular tube – has provision for light rays to enter the eyepieces or camera located above. This facilitates viewing along with photography.

 

Illumination

The light source, condenser, and objects are placed in the same plane.

Suffers from uneven lighting of the image.

The image shows hot spots, representing the coil of the bulb and uneven illumination.

Kohler or Kohler – type illumination:

Light microscopes with built-in illumination use such type of illumination.

There is an adjustable collector lens in front of the source of light[bulb] that focuses the source in the plane of the iris diaphragm of the condenser.

The object is evenly illuminated.

 

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