Microscopes Flashcards
Magnification
The degree of enlargement of an object compared with the actual object
Total magnification
The total magnification of an image seen through an optical light magnification is found by multiplying the magnification of the objective lens by the magnification of the eyepiece lens
Resolving power
The minimum distance that two close points can be seen as separate points. Theoretically half the wavelength of the light or electron beam, so 200nm in optical microscopes with blue light and excellent lenses, or 0.1nm with electron microscopes
Compound light microscope
A compound microscope with two lenses: eyepiece and objective. It is relatively inexpensive, portable and simple to use, although it has a limited wavelength (200nm)
Electron microscopes
A beam of electrons is focussed on the specimen with a condenser electromagnet. Parts that absorb electrons appear dark, while those that let them pass through appear lighter; an image is produced on a screen, which can be photographed. . The system must be in a vacuum to prevent scattering
Transmission electron microscope
High resolution microscopes (0.1nm) which require thin samples with different stains. You cannot directly see the image, as the system must be in a vacuum to prevent scattering. The system is expensive, and only produces a 2D image
Scanning electron microscope
A beam of electrons is focussed onto the specimen, and scattered by its surface. A 3D image is built up by passing the electron beam back and forth over the specimen. You can have thicker sample, with less preparation and setup needed, but you still need a vacuum. The image produced is lower resolution (200nm), and there is no image of the internal structure
Estimating object size
Place an eyepiece graticule into an eyepiece lens, and calibrate it against a stage micrometer (a 1cm scale with 100 divisions). You then estimate the size of the objects
