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Microscope and Microscopy - Permanent Slides-advanced

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Compound Microscope is an optical instrument used to observe objects too small for the naked eye. It uses two sets of lenses: the eyepiece (ocular) and the objective lens to achieve high magnification.

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Magnification is the process of enlarging the apparent size of an object. The total magnification is the product of the magnifying power of the objective lens and the eyepiece lens.

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Resolving Power (Resolution) is the ability of a microscope to distinguish two close points as separate entities. It is limited by the wavelength of light λ\lambda and the Numerical Aperture (NANA) of the lens system.

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Numerical Aperture (NANA) measures the light-gathering capacity of the objective lens. It is defined by the formula nsin⁡θn \sin \theta, where nn is the refractive index of the medium between the lens and the specimen.

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Permanent Slide Preparation involves several advanced steps: Fixation (preserving cell structure), Dehydration (removing water using ascending grades of alcohol like 30%30\%, 50%50\%, 70%70\%, 90%90\%, and Absolute Alcohol), Clearing (using Xylol), and Mounting.

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Mounting Media: For permanent slides, media like Canada Balsam or DPX (Dibutyl Phthalate Xylene) are used because they have a refractive index close to that of glass (n≈1.53n \approx 1.53), ensuring better clarity and long-term preservation.

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Staining (Differential Staining): Specific dyes are used to highlight different organelles. Safranin stains lignified tissues and plant cell walls red, while Methylene Blue is used for animal cell nuclei. Acetocarmine is frequently used to stain chromosomes during mitosis.

📐Formulae

Mtotal=Mobjective×MeyepieceM_{total} = M_{objective} \times M_{eyepiece}

R=0.61λNAR = \frac{0.61 \lambda}{NA}

NA=nsin⁡θNA = n \sin \theta

Limit of Resolution=1Resolving Power\text{Limit of Resolution} = \frac{1}{\text{Resolving Power}}

💡Examples

Problem 1:

A biology student is using a compound microscope equipped with a 10×10 \times eyepiece and a 45×45 \times high-power objective lens. What is the total magnification of the plant cell being observed?

Solution:

Mtotal=Meyepiece×MobjectiveM_{total} = M_{eyepiece} \times M_{objective} Mtotal=10×45=450×M_{total} = 10 \times 45 = 450 \times

Explanation:

To find the total magnification, multiply the magnifying power of the ocular lens (eyepiece) by the magnifying power of the objective lens currently in use.

Problem 2:

Calculate the Numerical Aperture (NANA) of an oil-immersion objective lens if the refractive index (nn) of the immersion oil is 1.511.51 and the semi-angle of the cone of light (θ\theta) entering the lens is 60∘60^\circ. (Use sin⁡60∘≈0.866\sin 60^\circ \approx 0.866)

Solution:

NA=nsin⁡θNA = n \sin \theta NA=1.51×sin⁡60∘NA = 1.51 \times \sin 60^\circ NA=1.51×0.866≈1.30766NA = 1.51 \times 0.866 \approx 1.30766 NA≈1.31NA \approx 1.31

Explanation:

The Numerical Aperture is a dimensionless number that characterizes the range of angles over which the system can accept light. Oil immersion increases 'n', thereby increasing 'NA' and improving resolution.

Problem 3:

During the preparation of a permanent slide of a dicot stem, a student mistakenly uses water as a mounting medium instead of DPX. Explain the likely consequence after one month.

Solution:

The slide will likely show signs of fungal growth or cellular decay, and the specimen will dry out due to evaporation.

Explanation:

Permanent slides require a non-volatile mounting medium like DPX or Canada Balsam. Water evaporates over time and promotes microbial growth, whereas DPX hardens, seals the specimen from air, and matches the refractive index of glass for permanent clarity.