1.3.1 Electron Microscopy

Introduction

The electron microscope is a powerful type of microscope that uses a beam of accelerated electrons instead of light to visualize specimens. It was developed because light microscopes have a limited resolution and cannot show the fine details of cell structures. The electron microscope has a much higher resolution and can magnify specimens millions of times.

Definition

Electron microscopy is a technique that uses a beam of accelerated electrons to visualize specimens and has a high resolution.

In simple words: An electron microscope uses electrons instead of light to see very tiny structures that a light microscope cannot show.

Core Concept

• Electron microscopes use electrons (not light) to form an image.
• They have a much higher resolution than light microscopes.
• They can magnify specimens 1 to 50 million times.
• They can only be used to view dead, dried specimens (not living specimens).
• The image produced is black and white.

Detailed Explanation

Working Principle of Electron Microscope

The working principle of an electron microscope is as follows:

• A beam of electrons is produced from an electron gun.
• The electron gun generates electrons and accelerates them.

• The beam of electrons is focused on the specimen through two sets of condenser lenses.
• An accelerating potential is applied between the filament and anode to move the electrons downwards.

• The specimen to be observed is kept as a thin section of 20–100 nm on a specimen holder.
• The specimen must be very thin so that electrons can pass through it.

• The beam of electrons goes through the specimen.
• Denser regions scatter more electrons on the screen than the lighter regions.
• This scattering creates contrast in the image.

• The beam of electrons scattering from the specimen passes through the objective lens to create a magnified image.
• This image is further magnified by the ocular lens.

• The final image is formed on a fluorescent screen or photographic plate.
• The image is black and white.

Key Components of an Electron Microscope

Magnification and Resolution

Why Electron Microscopes Have Higher Resolution

• Resolution depends on the wavelength of the illuminating source.
• Shorter wavelength = better resolution.
• Light has a wavelength of about 400–700 nm.
• Electrons have a wavelength of about 1 nm.
• Because electrons have a much shorter wavelength, electron microscopes can resolve much smaller details.
• This is why electron microscopes can see structures as small as 0.2 nm.

Why Electron Microscopes Can Only See Dead Specimens

• Electron microscopes operate in a vacuum.
• Living specimens cannot survive in a vacuum.
• The specimen must be dried and thin (20–100 nm).
• Therefore, only dead, dried specimens can be observed.

Why Electron Microscope Images Are Black and White

• Electrons do not have color.
• The image is formed by the scattering of electrons.
• Denser regions scatter more electrons (appear darker).
• Lighter regions scatter fewer electrons (appear lighter).
• This creates a black and white image.

Intermediate Level

The electron microscope was developed to overcome the limitations of the light microscope.

• Resolution limited to 250 nm.
• Cannot see fine details of organelles.
• Cannot see viruses or large molecules.

• Uses electrons with a wavelength of about 1 nm.
• Resolution of 0.2 nm.
• Can see the ultrastructure of cells (fine structure of organelles).

Types of Electron Microscopes:

There are two main types of electron microscopes:

• Electrons pass through the specimen.
• Used to see internal structures.
• Produces 2D images.

• Electrons scan the surface of the specimen.
• Used to see surface details.
• Produces 3D images.

Note: The textbook focuses on the general principle of electron microscopy. The two types are mentioned here to help your understanding, but the core concept is the same: electrons are used instead of light.

Important Points

• 1. Electron microscopy uses a beam of accelerated electrons to visualize specimens.
• 2. It has a high resolution compared to light microscopy.
• 3. The beam of electrons is produced from an electron gun.
• 4. The beam is focused on the specimen through two sets of condenser lenses.
• 5. An accelerating potential is applied between the filament and anode to move the electrons downwards.
• 6. The specimen is kept as a thin section of 20–100 nm on a specimen holder.
• 7. Denser regions scatter more electrons than lighter regions.
• 8. The scattered electrons pass through the objective lens to create a magnified image.
• 9. This image is further magnified by the ocular lens.
• 10. Magnification: 1 to 50 million times.
• 11. Resolution: 0.2 nm.
• 12. Image: Black and white.
• 13. Specimen: Only dead and dried specimens.
• 14. Vacuum: Essential for operation.
• 15. Comparison with light microscope:
• - Light microscope: light source, 1500X, live and dead specimens, colored image, 250 nm resolution, no vacuum.
• - Electron microscope: electron beams, 1–50 million times, dead specimens only, black and white image, 0.2 nm resolution, vacuum essential.

Remember

• - Electron microscopy = uses electrons, not light
• - Electron gun = produces electrons
• - Condenser lenses = focus the beam
• - Accelerating potential = moves electrons downwards
• - Specimen = thin section (20–100 nm)
• - Denser regions = scatter more electrons
• - Objective lens = forms first magnified image
• - Ocular lens = further magnifies
• - Magnification = 1 to 50 million times
• - **Resolution** = 0.2 nm
• - Image = black and white
• - Specimen = dead and dried only
• - **Vacuum** = essential
• - Light microscope = 1500X, colored, live/dead, 250 nm, no vacuum
• - Electron microscope = 1–50 million times, black and white, dead only, 0.2 nm, vacuum

Common Mistakes

• 1. Thinking electron microscopes can see living specimens – They cannot. They require a vacuum and dried specimens.
• 2. Thinking electron microscope images are colored – They are black and white.
• 3. Confusing magnification with resolution – Electron microscopes have both high magnification and high resolution, but these are different properties.
• 4. Forgetting that vacuum is essential – Light microscopes do not need a vacuum, but electron microscopes do.
• 5. Mixing up the resolution values – Light microscope = 250 nm; electron microscope = 0.2 nm.
• 6. Thinking electrons are the same as light – Electrons are particles with a much shorter wavelength than light, which is why they give better resolution.
• 7. Forgetting that denser regions scatter more electrons – This is how contrast is created in the image.
• 8. Thinking the specimen can be thick – The specimen must be very thin (20–100 nm) for electrons to pass through.
• 9. Confusing the order of lenses – Objective lens forms the first magnified image; ocular lens further magnifies it.

Quick Revision

• Electron microscopy uses a beam of accelerated electrons to visualize specimens.
• It has a high resolution (0.2 nm) compared to light microscopy (250 nm).
• Working principle:
• Magnification: 1 to 50 million times.
• Image: Black and white.
• Specimen: Only dead and dried specimens.
• Vacuum: Essential for operation.
• Comparison with light microscope:
• Light: light source, 1500X, live/dead, colored, 250 nm, no vacuum.
• Electron: electron beams, 1–50 million times, dead only, black and white, 0.2 nm, vacuum essential.

60-Second Revision

• Electron microscopy = uses electrons, not light
• Electron gun = produces electrons
• Condenser lenses = focus beam
• Specimen = thin section (20–100 nm)
• Denser regions = scatter more electrons
• Magnification = 1–50 million times
• **Resolution** = 0.2 nm
• Image = black and white
• Specimen = dead and dried only
• **Vacuum** = essential
• Warning: Electron microscopes cannot see living specimens. Images are black and white, not colored. Vacuum is essential.