Microscopes

Two main types of microscope

Light microscopes and electron microscopes

Light microscope – what lenses does it use?

A pair of convex glass lenses

Light microscope – resolution

0.2 µm

Why is light microscope resolution limited to 0.2 µm?

This is the wavelength of light, which restricts resolution

Electron microscope – resolution

0.1 nm

Magnification equation

Magnification = size of image ÷ size of real object

Resolution

The minimum distance apart that two objects can be distinguished as separate objects in an image

Effect of greater resolution

The image is clearer

Why are electron microscopes used?

To look at objects closer together than 0.2 µm, which light microscopes can't resolve

Two main types of electron microscope

Transmission electron microscope (TEM) and scanning electron microscope (SEM)

How do electron microscopes work?

A beam of electrons is focused by electromagnets inside a vacuum

Why do electron microscopes need a vacuum?

So particles in the air don't deflect electrons out of the beam alignment

How does a TEM work?

Electrons pass through a thin section of specimen; areas that absorb electrons appear darker on the micrograph

How does an SEM work?

Electrons pass across the surface and scatter; the scattering pattern builds a 3D image based on the specimen's contours

Electron microscope limitation – vacuum

Living specimens cannot be observed

Electron microscope limitation – staining

A complex staining process is needed, which may introduce artefacts

Electron microscope limitation – thickness

Specimens must be very thin, especially for TEM, so electrons can pass through

SEM vs TEM resolving power

SEM has lower resolving power than TEM, but both are greater than a light microscope