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AS Biology Revision Guide for OCR (2008 NEW course)
Unit 1: Cells, Exchange and Transport
Module 1: Cells
Light Microscopy and Magnification
Magnification is the degree to which the size of an image is larger than the
image itself. Numerically it is the image size divided by the actual size of the
object measured using the same units.
Resolution is the degree to which it is possible to distinguish between two
objects that are very close together. The higher the resolution the greater the
details you can see.
THE LIGHT MICROSCOPE
1. Light microscopes use a number of lenses to produce an image that can
be viewed directly at the eyepieces.
2. Light passes from a bulb under the stage through a condenser lens then
through the specimen.
3. This beam of light is focused through the objective lens then through the
4. To view specimens at different magnifications, light microscopes have a
number of objective lenses that can be rotated into position.
5. Usually four objective lenses are present x4, x10, x40 and x100. The x100
objective is an oil immersion lens.
6. The eyepiece lens then magnifies the image again. This is usually x10.
Advantages AND Limitations
Most light microscopes are capable of magnification of up to a maximum
The maximum resolving power using light is 200 nm. This means that if
two objects are closer together then 200 nm, they will be seen as one
object. This limit is due to magnitude of the wavelength of light. Two
objects can be distinguished only if light waves can pass between them.
A wide range of specimens can be viewed using a light microscope.
These include living organism such as EUGLENA and DAPHNIA.
Preparing a slide for a light microscope
1. STAINING Coloured stains are chemicals that bind to chemicals on or
in the specimen. This allows the specimen to be seen. Some stains bind
to specific cell structures. Acetic Orcein stains DNA dark red. Gentian
violet stains bacterial cell walls.
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SECTIONING Specimens are embedded in wax. Thin sections are then
cut without distorting the structure of the specimen. This is particularly
useful for making sections of soft tissue, such as brain.
Unit Symbol Metre Equivalent Fraction of a
Metre m 1 One
Decimetre dm 0.1 One tenth
Centimetre cm 0.01 One hundredth
Millimetre mm 0.001 One thousandth
Micrometre m 0.000001 One millionth
Nanometre nm 0.…read more