L3 Microscopy (Light Microscopes VS Electron Microscopes)

A Level Biology AQA

How does a light microscope work

• Light passes through specimen (light bounces off mirror)

• into magnifying lenses of microscope

• into objective viewing lenses

What is the max magnification of a light microscope?

1500 X

What is resolution?

The ability to distinguish between two points

What is a light microscope’s max resolution?

200nm

Why might a light microscope be more useful than an electron microscope in some cases?

• you can visualise specimens in their natural colour, rather than black and white

• more widely available, and easy to use

• you can use living cells and organisms, unlike electron microscopes which require them to be dead

• less expensive than electron microscopes

Label this diagram of a light microscope

How does an Electron Microscope work?

• Electrons have properties of waves and particles

• Starts with an electron gun, which is producing a beam of electrons

• these electrons pass down the microscope

• the inside of the microscope has a vacuum, so the electrons can pass through w/out bouncing off the molecules in the air

• electrons have a negative charge, so you can focus the electron beam using electromagnets

• these are called electromagnetic lenses

• the specimen is placed in the path of the electron beam. Electrons can pass through some parts of the specimen more easily than others

• Final image is produced on florescent screen

When was the electron microscope invented?

1931

What’s the electron microscopes maximum magnification?

TEM = around 50 million x

SEM = around 1,500,000 x

Compare resolution of Scanning and transmission electron microscopes

Transmission microscopes have a very high resolution, the scanning microscopes have a lesser resolution

What is the max resolution of an electron microscope?

around 20nm

Compare scanning and transmission electron microscopes magnification

TEM offers higher magnification, up to 50 million x

SEM is lower with up to 1-2million x

Compare scanning and transmission electron microscopes 2D VS 3D images

TEM offers 2D internal view, SEM offers 3D external view images

Compare scanning and transmission electron microscopes thickness of specimen that can be used

TEM= must be thinly sliced, SEM= not required to be thinly sliced

How does the transmission microscope work?

• Electron beam passes through the specimen

• In denser regions of the specimen, electrons are more easily absorbed, producing darker regions in the image

• must be performed in a vacuum

How does a scanning microscope work?

• Electron beam doesn’t pass through specimen, instead they are scattered from the surface of the specimen and detected.

• Producing a 3D image

• Requires specimen to be coated in metal such as gold —> leading to artefacts

What is an artefact (Electron microscope)? And what can cause them?

They are false images, and can be caused from the staining process, or the condition in the microscope