Electron microscope

Define an ‘electron microscope’

• Uses beams of electrons to magnify objects

Give 2 advantages of electron microscopes

• Electron beam has a very short wavelength ->> High resolving power

• As electrons are negatively charged, the beam can be focused using electromagnets

How does the electron microscope generally work?

• Because electrons are absorbed/deflected by molecules in air, a near-vaccumn has to be created within chamber of electron microscope in order for it to work effectively

What are the 2 types of electron microscope?

• Transmission electron microscope & scanning electron microscope

Resolution of transmission electron microscope

0.1 nm

How does the transmission electron microscope work?

• Consists of an electron gun that produces a beam of electrons that is focused into specimen by a condenser electromagnet

• Beam passes through thin section of specimen

• Parts of specimen absorb electrons & appear dark

• Other parts allow electrons to pass through & appear light

• Produces photomicrograph

How can the resolution of the transmission electron microscope be limited?

• Difficulties preparing specimen limit resolution achieved

• A higher energy electron beam is required & this may destroy the specimen

Give 4 main limitations of the transmission electron microscope

• Specimen must be extremely thin

• Complex staining process required & even then image is not in colour

• Whole system must be in vacuum therefore living organisms cannot be observed

• Image may contain artefacts

Give the limitations of scanning electron microscope

• Whole system must be in vacuum therefore living organisms cannot be observed

• Image may contain artefacts

• Lower resolving pattern than TEM, about 20nm

Why does the specimen need not be extremely thin for the scanning electron microscope?

• Electrons do not penetrate

How does the scanning electron microscope work?

• Directs beam of electrons into surface of specimen from above

• Beam passed back and fourth across specimen portion in a regular pattern

• Electrons scattered by specimen & pattern of scattering depends on contours of specimen surface

• Build up 3D image by computer analysis of pattern of scattered electrons & secondary electrons produced

Resolution of scanning electron microscope

• 20nm