Foundations in Biology

Magnification and Resolution

Magnification: The degree to which an image is larger than the object itself, numerically, it is the image size divided by the actual size of the object, measured using the same units. This is usually expressed as X10, X1.5 etc.

Magnification = Size of Image ÷ Actul size of specimen

Resolution is the degree to which it is possible to distinguish between two objects that are very close together. The higher resolution, the more detail.

The Light Microscope

- Use a number of lenses to produce an image that can be viewed directly at the eyepeice.

- Light comes from under the stage through the condenser lense and then through the specimen.

- The beam of light is then focused through the objective lense, then through the eyepeice lense.

-They have lenses that can be rotated.

- There are four lenses, X4, X10, X40, X100. The X100 is usually in oil immersion.

The Light Microscope (+'s and -'s)

Magnification is up to X1500.

Resolution- The Maximum resoling power using light is 200nm, meaning if 2 objects are closer than 200nm they will be seen as 1.

Specimens- Can view plant and blood cells and small animals such as Euglena and Daphnia.

Cant give internal cell structure.

Electron Microscopes

Scanning Electron Microscope (SEM)

The electron beam is directed onto a sample. The electrons don't pass through the specimen

They are 'bounced off' the sample.

The final image produced is a 3D view of the sufrace of the sample.

The magnification is X100,000 (Max)

Electron Microscopes

Transmission electron microscope (TEM)

The electron beam passes through a very thin prepared sample.

The electrons pass through the denser parts less easily, giving contrast.

2D image

The magnification is about X500,000 (Max)

Electron Microscopes

Advantages

The resolution is 0.1nm.

Can produce detailed images of organelles.

The SEM produces 3D images.

Disadvantages

Has to be in a vacuum.

Extremely expensive to buy.

Specimen has to be dead, high skill and training required to use them.