Question

Image formation in electron microscope is based on

• A. column length
• B. electron number
• C. differential scattering
• D. specimen size

Hint:

The fundamental principle of image formation in electron microscopy is that differences in how various parts of the specimen interact with and scatter the electron beam create contrast. This differential scattering allows us to visualize the internal structure (in TEM) or surface topography (in SEM) of the sample.

Answer 1

The Correct Option is C

Image Formation in Electron Microscopy

Image formation in an electron microscope (such as Transmission Electron Microscopy (TEM) or Scanning Electron Microscopy (SEM)) relies on the interaction of a beam of electrons with a sample to produce a detailed image of its structure or surface. Unlike optical microscopes that use light, electron microscopes use electrons, which have a much shorter wavelength, enabling higher resolution. 

Let’s evaluate each option.

1. Column Length

The column length refers to the physical length of the electron microscope’s column, which houses components like the electron gun, lenses, and detectors. While the column’s design is critical for aligning and focusing the electron beam, it is not directly responsible for image formation. Image formation depends on how electrons interact with the sample, not the length of the column. Thus, this option is incorrect.

2. Electron Number

Electron number could imply the quantity or intensity of electrons in the beam. While the number of electrons affects the signal strength or image brightness, it is not the fundamental mechanism of image formation. The image is created based on how electrons are altered (scattered or transmitted) by the sample, not merely their quantity. This option is not the primary basis for image formation.

3. Differential Scattering

Differential scattering refers to the varying ways in which electrons interact with different parts of the sample. In TEM, electrons pass through a thin sample, and their scattering (elastic or inelastic) depends on the sample’s composition, thickness, and atomic structure. Areas with higher atomic number or density scatter electrons more, creating contrast in the image. In SEM, scattered electrons (e.g., backscattered or secondary electrons) provide surface information. This differential scattering of electrons by the sample’s structure is the core mechanism for forming images in electron microscopy, making this option correct.

4. Specimen Size

Specimen size refers to the physical dimensions of the sample being imaged. While the sample must be appropriately sized (e.g., very thin for TEM or small enough for SEM’s stage), image formation is not based on the size itself. Instead, it depends on how electrons interact with the sample’s material properties. Specimen size is a practical consideration, not the mechanism of image formation, so this option is incorrect.

Why Differential Scattering is the Correct Answer

Image formation in an electron microscope is based on differential scattering. When the electron beam interacts with the sample, electrons are scattered differently depending on the sample’s atomic composition, density, and structure. In TEM, for example, electrons passing through a biomaterial sample (e.g., a protein nanostructure) are scattered variably: denser areas scatter more electrons, appearing darker in the image, while less dense areas allow more electrons to pass, appearing brighter. In SEM, differential scattering of backscattered or secondary electrons reveals surface topography. This contrast created by differential scattering forms the basis of the image. For instance, in studying a biomaterial like bone, differential scattering highlights variations in mineral density, enabling detailed imaging.

Why Not the Other Options?

• Column Length: Affects the microscope’s design but not the mechanism of image formation.
• Electron Number: Influences signal intensity but not the fundamental process of image creation.
• Specimen Size: A practical requirement, not the basis for how the image is formed.

A Simple Way to Remember

Think of an electron microscope as painting a picture with electrons:

• Differential Scattering: The “brushstrokes” vary based on the sample’s texture (structure), creating the image’s contrast.
• Column Length: The size of the easel, not the painting process.
• Electron Number: The amount of paint, affecting brightness but not the picture’s details.
• Specimen Size: The size of the canvas, not how the picture is drawn.

Final Answer

Image formation in an electron microscope is based on Differential Scattering.