Magnification and General Laboratory Measurements

Choosing a magnification

Biological specimens vary greatly in size. A hand lens can enlarge a whole leaf or a small insect so that surface features are easier to see. A light microscope is useful for much smaller structures, such as cells on a prepared slide. You can investigate magnification by viewing specimens with a hand lens and then with different microscope objective lenses, noting which features become visible at each setting. Start a microscope observation on a low-power objective: it gives a wider view in which to locate the specimen before moving to higher power.

A light microscope magnifies through two lenses. Its total magnification is the eyepiece magnification multiplied by the objective magnification. Higher magnification makes the image larger, but it usually shows a smaller part of the specimen at once. Magnification is not the same as resolution: enlarging a blurred image will not reveal details that were never resolved.

You may also need to find the magnification of a photograph or scientific diagram. Compare the size of a feature in that image with its actual size:

$\text{image magnification}=\frac{\text{image size}}{\text{actual size}}$

Measure the same feature in both cases and convert the lengths to the same unit before dividing. Magnification is a ratio, so it has no unit. The magnification of a printed image may differ from the microscope setting because the image can be enlarged or reduced after it is taken. For very small specimens, remember that $1\text{ mm}=1000\text{ µm}$.

Making measurements that can be compared

An observation becomes more useful when it includes an accurate measurement and its unit. Choose apparatus with a suitable range and scale: a scale marked too coarsely will not distinguish small differences. Read scales from directly in front to avoid a change in apparent position called parallax, and record what the instrument actually shows rather than adding unsupported decimal places.

Use a ruler or an eyepiece scale to measure length, depending on the size of the specimen; record it in an appropriate unit such as mm or µm. Area describes the amount of surface covered and is recorded in square units, such as cm² or mm². For a regular rectangle, measure its length and width and multiply them. For an irregular shape, a grid can give an estimate by counting whole squares and combining partly covered squares.

Measure mass with a balance, in g or another stated mass unit. Zero the balance first; if the specimen needs a container, use the tare function so the reading is for the specimen alone. Measure time with a stopwatch, usually in seconds, starting and stopping it at clearly defined events. Measure temperature with a thermometer or temperature probe, usually in °C, allowing the reading to settle before recording it.

A measuring cylinder is suitable for many liquid volumes. Stand it upright on a level surface and read the bottom of the curved meniscus at eye level, using a unit such as cm³ or mL. A gas volume can be measured with a gas syringe attached to an apparatus that collects the gas; read the scale at the syringe piston and check that the connections do not leak. To measure pH, compare the colour of pH paper or universal indicator with its chart, or use a calibrated pH probe for a numerical reading. pH is a number without a unit.

In any investigation, record measurements in a table with a heading and unit for each quantity. This makes results from different specimens, magnifications or conditions easier to compare. OCR’s microscopy practical activity group includes accurate measurement of length, area, mass, time, temperature, volumes of liquids and gases, and pH; these techniques may be practised in different investigations across the course.