electron microscopy

why do light microscopes have poor resolution?

due to the long wavelength of light rays

Advantage of electron microscope?

• short wavelength, so high resolution

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

why does a near vacuum have to be created?

because electrons are absorbed or deflected by molecules in the air

types of electron microscope

• transmission electron microscope

• scanning electron microscope

what does the tem consist of ?

an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet

what happens next? Why?

the beam passes through a thin section of specimen. Parts of the specimen absorb electrons and become dark. Other parts allow electrons to pass through and therefore appear bright.

what is the term called when a image produced on the screen is photographed?

photomicrograph

what is the resolving power of the TEM

0.1 nano metres

Why cant the resolving power always be achieved in practice?

• difficulties preparing the specimen limit the resolution that can be achieved

• high energy beam is required and this may destroy the specimen

Describe the main limitations of a TEM

• the whole system must be in a vacuum and the living specimen cannot be observed

• the specimen must be extremely thin

• a complex staining process needed but the image is not in colour

• the image may contain artefacts

what are artefacts?

are things that result from the way the specimen is prepared. May appear on the photomicrograph but not on the natural specimen. Hard to know what to expect.

why must the specimen be thin?

to allow electrons to penetrate, to allow a 2D image.

how can a 3D model be created?

taking a series of sections, and build up a 3D image of specimen by looking at the series of photomicrographs. This is slow and complex and can be fixed by SEM

why does the specimen not required to be extremely thin in a SEM?

electrons do not penetrate

what does the scattering of electrons depend on?

the contours of the specimen surface.

how can a 3D image be built?

by a computer analysis of the pattern of scattered electrons and secondary electrons produced.

what is the simple difference between SEM and TEM?

the SEM has a lower resolving power than TEM