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Microscope and Microscopy - Let us compare Light microscope and Electron microscope (TEM and SEM)-advanced

Grade 9CBSE

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

🔑Concepts

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Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye. The two most important parameters are Magnification (the ratio of an object's image size to its real size) and Resolving Power (the minimum distance two points can be separated and still be distinguished as separate points).

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The Light Microscope (Compound Microscope) uses visible light as the source of illumination and glass lenses to focus light. Its resolution is limited by the wavelength of visible light (400 nm400\text{ nm} to 700 nm700\text{ nm}), reaching a maximum resolution of about 200 nm200\text{ nm} and magnification up to 2000×2000\times.

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The Electron Microscope uses a beam of electrons instead of light. Since the wavelength of electrons is significantly shorter than visible light (approx. 100,000100,000 times shorter), the resolving power is much higher, reaching 0.2 nm0.2\text{ nm} to 10 nm10\text{ nm}.

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Transmission Electron Microscope (TEM) aims a beam of electrons through a very thin section of the specimen. It is primarily used to study the internal ultrastructure of cells. The image produced is 2D.

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Scanning Electron Microscope (SEM) scans the surface of a specimen coated with a thin gold film. It excites electrons on the surface, which are detected to create a 3D image of the specimen's topography.

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Comparison of Source: Light Microscope uses Photons (light), while Electron Microscopes use Electrons. Comparison of Lenses: Light Microscope uses Glass lenses, while Electron Microscopes use Electromagnetic lenses.

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Specimen Preparation: In Light Microscopy, living cells can be observed. In Electron Microscopy, specimens must be fixed, dehydrated, and placed in a vacuum, meaning only dead organisms can be viewed.

📐Formulae

Total Magnification=Magnification of Objective Lens×Magnification of Ocular Lens\text{Total Magnification} = \text{Magnification of Objective Lens} \times \text{Magnification of Ocular Lens}

d=0.61λNAd = \frac{0.61 \lambda}{\text{NA}}

Limit of Resolution (d)∝λ (Wavelength)\text{Limit of Resolution (d)} \propto \lambda \text{ (Wavelength)}

λe≈1.23V nm\lambda_e \approx \frac{1.23}{\sqrt{V}} \text{ nm}

💡Examples

Problem 1:

A student is using a compound light microscope with a 10×10\times ocular lens and a 45×45\times objective lens. Calculate the total magnification of the specimen being observed.

Solution:

Given: Mocular=10×M_{\text{ocular}} = 10\times Mobjective=45×M_{\text{objective}} = 45\times

Using the formula: Total Magnification=Mocular×Mobjective\text{Total Magnification} = M_{\text{ocular}} \times M_{\text{objective}} Total Magnification=10×45=450×\text{Total Magnification} = 10 \times 45 = 450\times

Explanation:

The total magnification is the product of the individual magnifications of the lenses in the light path.

Problem 2:

Why does an Electron Microscope provide a higher resolution than a Light Microscope? Compare their resolving limits given λlight≈500 nm\lambda_{\text{light}} \approx 500\text{ nm} and λelectron≈0.005 nm\lambda_{\text{electron}} \approx 0.005\text{ nm}.

Solution:

Resolution dd is directly proportional to the wavelength λ\lambda. For Light Microscope: d≈200 nmd \approx 200\text{ nm} (using λ≈500 nm\lambda \approx 500\text{ nm}). For Electron Microscope: d≈0.2 nmd \approx 0.2\text{ nm} (using λ≈0.005 nm\lambda \approx 0.005\text{ nm}).

Ratio of Resolution: dlightdelectron=200 nm0.2 nm=1000\frac{d_{\text{light}}}{d_{\text{electron}}} = \frac{200\text{ nm}}{0.2\text{ nm}} = 1000

Explanation:

Because electrons have a much shorter wavelength than visible light, they can resolve structures that are 1000 times smaller than what a light microscope can see.