2- Magnification and resolution

What is magnification?

• A ratio

• Number of times larger ab image appears compared to the size if the actual object

• It doesn’t effect detail

What is resolution?

• The ability to distinguish between structures

• Limited by differentiation of light as it passes through samples and objects

• Can be increased by using beams of electrons instead of light as electrons have a shorter wavelength

Working out magnification

Actual size= image size / magnification

• Cells normally measured in micrometers (μm)

• Ultrastructure usually measured in nano metes (nm)

Units of measurement

The unit of measurement must be appropriate to the object being measured

Calibrating a microscope

• To measure the size of a sample under a microscope it needs to be calibrated

• True magnification of different lenses varies so every microscope needs to be calibrated, lenses too

• This is done by using an eyepiece graticule and stage micrometer

• A microscope eyepiece is fitted with a graticule

Stage micrometer

• Placed on microscope stage

• Ruler is 1mm (1000μm) long with 100 divisions so each division is 10 μm

Calculating conversion factor

You havre to do this for each objective lens you use

On x40:

1mm =40 EPUs

1000μm= 40 EPUs

1000/40 =25 25

Each division = 25 μm

On x100:

1mm =100 EPUs

1000μm= 100 EPUs

1000/100=10

Each division = 10μm

On x400:

0.25 mm=100 EPUs

250μm= 100 EPUs

250/100 =2.5

Each division = 2.5 μm

EPU- eyepiece unit

Once calibrated

• Remove stage micrometer and replace with specimen slide

• Don’t change the lens

• Line up specimen with graticule lines and count divisions

• Multiply by the conversion factor (what each division is worth in μm)