Electron Microscope

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Slide 1

AS Biology Core Principles

The Electron Microscope

Slide 2

Aims

Resolving power

The resolving power of light & electron microscopes

The difference between the light & electron microscope

Transmission & scanning electron microscopy

Slide 3

Introduction

Microscopes magnify & resolve images

Microscopy began in 1665 when Robert Hooke coined the word ‘cells’ to describe the structure of cork

You need to know about 2 types of microscope - light & electron

You need to know how they work and the differences between them

‘Its not how much they magnify that is key - but how well they resolve…’

Slide 4

Resolving Power

The limit of resolution of a microscope is the smallest distance between 2 points that can be seen using a microscope

This is a measure of the clarity of the image

A microscope with a high resolving power will allow 2 small objects which are close together to be seen as 2 distinct objects

Slide 5

Resolving Power

Resolving power is inversely proportional to the wavelength of the radiation it uses

Slide 6

The Light Microscope

Series of lenses through which ordinary white light can be focused

Optical microscopes can not resolve 2 points closer together than about half (0.45) the wavelength of the light used (450-600nm)

How close is this?

Slide 7

The Light Microscope

The total magnification is the eyepiece magnification multiplied by the objective magnification

The maximum magnification of a light microscope is x1500

What can it be used for?

What can it not be used for?

Slide 8

The Electron Microscope

Electrons (negatively charged, very small particles) can behave as waves

The wavelength of electrons is about 0.005nm

What will this mean for the limit of resolution?

Electrons are ‘fired’ from an electron gun at the specimen and onto a fluorescent screen or photographic plate

Where is this technique commonly used?

There are 2 types of electron microscopy - transmission and scanning

Both focus an electron beam onto the specimen using electromagnets

Slide 9

Transmission Electron Microscope (TEM)