Microscopes/Magnification Flashcards
Magnification equation
Image/Object
Remember to convert to correct units
Magnification
Being able to enlarge something in appearance.
Resolution
Ability to distinguish points close together.
The two types of electron microscopes
Transmission electron microscopes, Scanning electron microscopes
Transmission electron microscope
They produce a black and white image. It results in a 2-D image. It consists of an electron gun that produces a beam of electron that is focused onto the specimen by a condenser electromagnet. The beam passes through a thin section of the specimen. Parts of the specimen absorb electrons and therefore appear dark. Other parts allow the electrons to pass through so they appear bright. The image produces a photomicrograph.
The main limitations of TEM
The whole system must be in a vacuum so living specimens cannot be observed. A complex staining process is required. The specimen must be extremely thin to allow the electrons to penetrate the specimen. This results in a 2-D image. The image may contain artefacts.
Scanning electron microscopes
It produces a 3-D image. The SEM directs a beam of electrons onto the specimen from above rather than from below like a TEM. The electrons are scattered by the specimen. Computer analysis allows a 3-D image to be seen.
Benefits of electron microscopes over light (optical) microscopes
They have a higher resolution than light microscopes because the electron beam has a shorter wavelength. Therefore it can see smaller objects than light microscopes because of a higher resolving power.
What should the object be in a light microscope?
The object should be thin to be viewed under a microscope. It should be a single layer of cells thick to allow the light to pass through.
The eye piece graticule
It is a glass disc fitted into the eyepiece of the microscope. The absolute size is not important as this is what will be calibrated.
The stage micrometer
It I used to calibrate the eyepiece graticule. A stage micrometer consists of a microscope slide on which is engraved a fine and accurate scale. This is used to calibrate the eyepiece graticule.
Stages of using an eyepiece graticule (e.g. using stomata)
Measure each stoma using the eyepiece graticule.
Calibrate the eyepiece graticule against the stage micrometer using a ruler
Take a number of measurements to calculate the mean.
Benefits of a light microscope over an electron microscope
It can look at living cells because they don’t have to be in a vacuum.
They can look at colour images.
Artefacts
These are things that result from the way the specimen is prepared. They are not part of the natural specimen.
