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Microscope and Microscopy - Electron Microscopes-advanced

Grade 9CBSE

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

🔑Concepts

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An Electron Microscope (EM) is a type of microscope that uses a beam of accelerated electrons as a source of illumination, providing much higher resolution than light microscopes.

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The resolving power of a microscope is inversely proportional to the wavelength (λ\lambda) of the radiation used. Since electrons have wavelengths about 100,000100,000 times shorter than visible light, they can resolve structures as small as 0.10.1 to 0.2 nm0.2 \text{ nm}.

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Transmission Electron Microscope (TEM): Used to view the internal structure of cells. Electrons pass through an ultra-thin section of the specimen.

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Scanning Electron Microscope (SEM): Used to study the surface topography of a specimen. It provides a 33D-like image by scanning the surface with a focused electron beam.

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Electromagnetic Lenses: Unlike light microscopes that use glass lenses, electron microscopes use electromagnetic coils to focus the electron beam.

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Vacuum Condition: Electron microscopes must operate under a vacuum because air molecules would scatter the electrons, preventing them from reaching the specimen.

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Specimen Preparation: Living specimens cannot be observed in an EM because the vacuum and high-energy electron beam are lethal. Specimens must be dehydrated and often coated with a thin layer of metal like gold.

📐Formulae

M=Size of ImageActual Size of ObjectM = \frac{\text{Size of Image}}{\text{Actual Size of Object}}

d=0.61λNAd = \frac{0.61 \lambda}{NA}

λ≈1.23V nm\lambda \approx \frac{1.23}{\sqrt{V}} \text{ nm}

Resolving Power∝1λ\text{Resolving Power} \propto \frac{1}{\lambda}

💡Examples

Problem 1:

A ribosome has an actual diameter of 20 nm20 \text{ nm}. If it is viewed under an electron microscope at a magnification of 500,000×500,000 \times, calculate the diameter of the image produced in millimeters (mmmm).

Solution:

Given: Actual size =20 nm=20×10−6 mm= 20 \text{ nm} = 20 \times 10^{-6} \text{ mm} Magnification (MM) =500,000= 500,000 Using the formula: Image Size=M×Actual Size\text{Image Size} = M \times \text{Actual Size} Image Size=500,000×20 nm\text{Image Size} = 500,000 \times 20 \text{ nm} Image Size=10,000,000 nm\text{Image Size} = 10,000,000 \text{ nm} Since 1 mm=1,000,000 nm1 \text{ mm} = 1,000,000 \text{ nm}: Image Size=10,000,0001,000,000=10 mm\text{Image Size} = \frac{10,000,000}{1,000,000} = 10 \text{ mm}

Explanation:

The image of the ribosome will appear as 10 mm10 \text{ mm} (or 1 cm1 \text{ cm}) in diameter under the microscope.

Problem 2:

Compare the resolving power of a light microscope (using λ=500 nm\lambda = 500 \text{ nm}) and an electron microscope (using λ=0.05 nm\lambda = 0.05 \text{ nm}) assuming the Numerical Aperture (NANA) is constant.

Solution:

Resolving power is inversely proportional to the wavelength (d∝λd \propto \lambda). For light microscope: dlight∝500 nmd_{light} \propto 500 \text{ nm} For electron microscope: delectron∝0.05 nmd_{electron} \propto 0.05 \text{ nm} Ratio of resolution limits: dlightdelectron=500 nm0.05 nm=10,000\frac{d_{light}}{d_{electron}} = \frac{500 \text{ nm}}{0.05 \text{ nm}} = 10,000 Since the resolution limit (dd) of the EM is 10,00010,000 times smaller than that of the light microscope, its resolving power is 10,00010,000 times higher.

Explanation:

A smaller resolution limit means the microscope can distinguish between two points that are much closer together, resulting in a clearer, more detailed image.