magnification and calibration.

What’s magnification?

How many times larger an image is than the actual size of the specimen

What’s resolution?

The ability to distinguish between two close together objects.

How can you increase resolution?

By using beams of electrons which have a wavelength thousands of time shorter then light. This means that individual beams can be much closer before they overlap.

What’s the calculation for magnifications?

Image size / actual size X magnification

How do you measure the size of a sample under a microscope?

Using a graticule.

Why do we need a graticule?

The true magnification of the different lenses of a microscope can vary slightly so every microscope needs to be calibrated individually using the scale on the graticule in the eyepiece. The eyepiece graticule has no units and remains unchanged with different objective lens. This means that microscopes need to be calibrated at each different magnification used.

How do we use the graticule?

A stage micrometer is a slide with a very accurate scale in micrometers etched into it which is used to to calibrate the eyepiece graticule.

1) line up the eye piece graticule with the stage graticule and focus the microscope

2) count the number of eyepiece divisions and note the number of micrometres between the points of coincidence

3) move the slide to line up the zeros on the two scales . Then look along the scales for lines that concise

4) each eyepiece division= no of eyepiece division/ number of micrometers