Magnification and Resolution
Magnification - the degree to which the size of the image is larger than the object itself
Resolution - the degree to which it is possible to distinguish between two objects that are very close together, the higher the resolution the greater the detail you can see
- Use a number of lenses to produce an image that can be veiwed directly at the eye peice
- Light passes from the bulb, through a conditioner lense and then through the specimen
- The beam of light is focussed through the objective lense, then through the eye peice
- You can veiw specimens at different magnification, as light microscopes have number of different lenses which can be rotated
- There are usually 4 objective lenses: x4 x10 x40 and x100
- The eyepeice then magnifies this again which is usually x10
- The total magnification = objective lense x eyepeice lense
Advantages and Limitations of Light Microscopes
Magnification - most are capable of a maximum of x1500
Resolution - Maximum is 200nm. Meaning if two objects are closer together than 200nm they will be seen as only one object. This is due to the magnitude of the wavelength of light.
Specimens - wide range of specimens can be veiwed using a light microscope
Used widely in education, labotory analysis and research. However as it doesn't have a high resolution it can't give detail about internal cell structure
Preparing Specimens for a Light Microscope
- Staining: coloured stains are chemicals that bind to chemicals on or in the specimen, it allows the specimen to be seen. Some stains bind to specific cell structures Eg. Acetic Orcein stains DNA dark red and Gentain Violet stains bacterial walls.
- Sectioning: Specimens are embedded in wax, thin sections are then cut without distorting the structure of the specimen. Particularly useful for making sections of soft tissue.
Suitable Units of Measurements
- The metre - divided into 1000mm, because cells and organelles are too small they cannot be measured in m's. Most animals cells are 20-40 millionths of a metre, so the micrometre has to be used.
- The micrometre - 1000 micrometres are 1mm. 1 micrometre is a millionth of a metre, animal cells are usuallt 20 - 40 micrometres long.
- The nanometre - Some biological structures are too small even the micrometre is too small. 1000 nanometres are 1 micrometre and a nanometre is a thousand millionths of a metre. Cell surface membranes are around 10 nm wide.
Limits of resolution
The resolution for the:
Human eye - 100 micrometres
Light microscope - 200 nanometres
Electron microscope - 0.2 nanometres
Acheive a high resolution
- Generate a beam of electrons (of which the wavelength is 0.04nm)
- Distinguish objects 0.2nm apart
- Use magnets to focus beam of electrons onto prepared specimen
- Electrons not visible to the human eye, image produced is projected onto screen/photgraphic paper to make a black and white image (called electron micrgraphs)
- Resolution is 500000 times greater than the human eye
Types of Electron Microscope
- Transmission Electron Microscope - electron beam passes through a very thin prepared sample. Passes through the denser part of the sample less easily so gives so contrast, final image produced is two dimensional. Magnification possible is 500000
- Scanning Electron Microscope - The electron is projected onto a sample, it doesnt pass through the specimen, they are then bounced off the sample. The final image is the 3D surface of the sample. The magnification is about 100000.
Advantages and Limitation of Electron Microscopes
Advantages: resolution is 0.1nm, can be used to produce detailed images of the structures inside the cell. The SEM can produce 3D images
Disadvantages: Electron beams are deflected by the molecules in air, they are also extremely expensive. Preparing samples and using this microscope requires a lot of skill and training.