Microscopes and magnification

What is meant by magnification?

• The number of times larger an image appears compared with the size of the object

What is meant by resolution?

• The distance between 2 distinct points on image

What is an photomicrograph?

• Photograph of an image seen using an optical microscope

What is an electron micrograph?

• Photograph of an image seen using an electron microscope

What type of magnification do microscopes produce?

• Linear - This means that if a specimen is magnified x100, it appears 100x wider and longer than it really is

Why are optical microscopes used in schools, hospitals, and colleges?

• Because they are relatively cheap

• They are easy to use

• They are portable

• They are able to study living specimen as well as dead

What is the maximum magnification of an optical microscope?

• x1500

Why is the resolution in an optical microscope limited?

• This is because they use visible light which has a wavelength of between 400 - 700 nm, therefore structures closer than 200nm(0.2um) will appear as one object

What are the advantages and disadvantages of an optical microscope?

Adv:

• Relatively cheap

• Easy to use

• Portable

• Can look at dead and live specimen

• Can produce a Coloured image

Disadvantages:

• Limited resolution

• Limited Magnification

• Image is only 2d

What do electron microscopes use to produce images instead of light?

• They use fast travelling electrons with a wave length of 0.004nm

How does using electrons change the image?

• It significantly increases the magnification

• It increases the resolution so that 2 points 0.5 nm apart can be distinguished as separate points

How much greater is the resolving power of an electron microscope than that of an optical microscope?

• 400x greater

What are the 2 types of electron microscopes?

• Transmission electron microscopes

• Scanning electron microscopes

Describe the process of how a transmission electron microscope produces an image?

• The specimen has to chemically fixed by being dehydrated and stained with metal salts

• The specimen also has to be cut extremely thin

• The beam of electrons is shot out of the source and passes over a condenser lens then onto the specimen

• Some electrons pass through and some are focused on the screen of photographic plate

• the electrons then form a 2d black and white (grey scale) image

What is the maximum magnification and resolution produced by transmission electron microscopes?

• x500,000

• 0.2nm

How is an image produced by an scanning electron microscope?

• Electrons are fired at the specimen but do not pass through it, but instead secondary electrons bounce off the specimen’s surface and then focus onto a screen

• This gives a 3D grey scale image

What is the maximum resolution and magnification of a scanning electron microscopes?

• x100,000

• 0.2nm

What are the advantages and disadvantages of a transmission electron microscope?

Adv:

• x500,000 magnification - very high

• 0.2nm resolution - very high

Disadv:

• Very large and expensive

• Need a lot of skill and training to use them

• Image has no colour (grey scale)

• Specimen cant be alive, must be in a vacuum

• Stains can be hazardous to user

What are the advantages and disadvantages of a scanning electron microscope?

Adv:

• Image is 3d

• x100,000 magnification - Very high

• 0.2nm resolution - Very high

Disadv:

• Very large and expensive

• Need a lot of skill and training to use them

• Specimen cant be alive, must be in a vacuum