WJEC/Eduqas A Level Biology A Student Guide to Practical Work

• • • Practical tip • • • Line up the zeros on the stage micrometer and the eyepiece graticule absolutely exactly. Grade boost Remember to include a key for abbreviations: smu = stage micrometer unit epu = eyepiece unit • • • Practical tip • • • Many school microscopes have ×4, ×10 and ×40 objectives. The calibrations generally work out approximately as shown in this table. Objective magni cation Length of 1 epu /µm 4 25 10 10 40 2.5 To know what length each division represents, you need a stage micrometer. This is a microscope slide on which the object is a line 1mm long. It is ruled with markings for tenths and hundredths of a millimetre: Line up at the zero of the eyepiece graticule and the zero of the stage micrometer, making sure the scales are absolutely parallel. Look along the scales and see where they coincide again. E.g. on a ×40 objective, 20 stage micrometer divisions may line up exactly with 80 eyepiece divisions: Now three statements for the sums, remembering to say which objective you have used: At a ×40 objective lens 1. 80 eyepiece units = 20 stage micrometer units ∴ 1 epu = 20/80 smu = 0.25 smu 2. 1 smu = 0.01 mm 3. 1 epu = 0.25 × 0.01 mm = 0.0025 mm Very small numbers of mm are better expressed as microns or micrometres (µm), so you can complete the calculation like this: = 0.0025 × 1000 µm = 2.5 µm You can work out similar calibrations for all the objectives that you use. Microscopes differ slightly in their calibration so you have to do it again for each microscope you use. That is why, in school, it helps if you always select the same microscope. Then you only have to do it once. We will see how to use the calibration in measuring when we discuss biological drawings. 1mm 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 10 20 60 70 80 0 1 2 3 4 5 6 7 8 9 10 ×40 objective 9 A.1 Microscopy