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Microscope and Microscopy - New Developments-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. Advanced microscopy is crucial for understanding the ultra-structure of cells.

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Magnification is the process of enlarging the appearance of an object. Total magnification in a compound microscope is calculated as the product of the ocular lens and the objective lens powers: Mtotal=Mocular×MobjectiveM_{\text{total}} = M_{\text{ocular}} \times M_{\text{objective}}.

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Resolving Power (Resolution) is the ability to distinguish two close points as separate entities. For a light microscope, the limit of resolution is approximately 0.2μm0.2 \mu m (micrometers).

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Electron Microscope (EM): Developed by Knoll and Ruska (1931), it uses a beam of electrons instead of light and electromagnetic lenses instead of glass lenses. The wavelength of electrons is much shorter than visible light, allowing for higher resolution (0.10.1 to 10 nm10 \text{ nm}).

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Transmission Electron Microscope (TEM): Used to view the internal ultra-structure of cells (organelles). It provides a 2D image. Magnification can reach up to 500,000×500,000\times.

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Scanning Electron Microscope (SEM): Used to study the 3D surface topography of specimens. The specimen is usually coated with a thin layer of gold or platinum.

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Phase Contrast Microscopy: A technique that allows the visualization of living, unstained cells by exploiting differences in the refractive index of different cell parts.

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Fluorescence Microscopy: Uses high-intensity light to excite fluorescent molecules (fluorophores) in a specimen, which then emit light of a longer wavelength. This is used for tracking specific proteins or structures.

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Limit of Resolution (dd): This is inversely proportional to the Numerical Aperture (NANA) of the lens. A smaller dd means higher resolution.

📐Formulae

Mtotal=Mocular×MobjectiveM_{\text{total}} = M_{\text{ocular}} \times M_{\text{objective}}

Actual Size=Image SizeMagnification\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}}

d=0.61λNAd = \frac{0.61 \lambda}{NA}

1μm=10−6m1 \mu m = 10^{-6} m

1nm=10−9m1 nm = 10^{-9} m

💡Examples

Problem 1:

A student uses a compound microscope with an ocular lens of 15×15\times and an objective lens of 40×40\times. What is the total magnification? If a cell appears to be 6mm6 mm long under this microscope, what is its actual size?

Solution:

  1. Total Magnification: Mtotal=15×40=600×M_{\text{total}} = 15 \times 40 = 600\times
  2. Actual Size: Actual Size=Image SizeMagnification\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}} Actual Size=6mm600\text{Actual Size} = \frac{6 mm}{600} Actual Size=0.01mm=10μm\text{Actual Size} = 0.01 mm = 10 \mu m

Explanation:

The total magnification is the product of the magnifying powers of the two lenses. To find the actual size, the observed image size is divided by the total magnification factor.

Problem 2:

Why can an electron microscope resolve objects as small as 0.2nm0.2 nm, while a light microscope is limited to 200nm200 nm?

Solution:

The resolution (dd) depends on the wavelength (λ\lambda) of the radiation used: d∝λd \propto \lambda Visible light has wavelengths between 400nm400 nm and 700nm700 nm. Electrons, when accelerated, act as waves with extremely short wavelengths (approximately 0.005nm0.005 nm). Because the wavelength of electrons is significantly smaller than that of visible light, the limit of resolution is much lower, allowing for much higher detail.

Explanation:

Resolution is limited by diffraction, which is a function of wavelength. Shorter wavelengths allow for the imaging of smaller structures.