Using a Microscope
Use a light microscope correctly to prepare, focus, and examine biological specimens, including making accurate labelled drawings at an appropriate magnification
What a learner can do afterwards
- Sets up a light microscope safely and correctly (coarse focus then fine focus)
- Prepares a wet mount slide with a biological specimen (e.g. onion skin)
- Calculates the magnification of an image (magnification = image size ÷ actual size)
- Draws an accurate labelled diagram of observed cells with a scale
1 · Read
Cheek cells are too small to see clearly by eye. A school light microscope uses an eyepiece and an objective to enlarge them. Start at the lowest power. Watch the gap from the side while bringing the lens and slide closer; do not let them touch. Through the eyepiece, use coarse focus to increase the gap slowly until an image appears. Then sharpen it with fine focus. Use only fine focus at high power.
With your teacher, peel a thin inner layer from an onion. Lay it flat on a clean slide and add a drop of water. Touch one edge of a coverslip to the water, then lower the other edge slowly to reduce trapped air bubbles. Iodine stain can help cell parts stand out. Use stains only as your teacher directs.
Use matching units. Magnification = image size ÷ actual size. A 9 mm drawing of a 0.03 mm cell has magnification 9 ÷ 0.03 = 900 ÷ 3 = 300. Multiplying both sizes by 100 leaves the ratio unchanged. To find image size, use image size = magnification × actual size.
Draw only the parts you observe. Use a sharp pencil and straight ruled label lines. Include a scale line (scale bar) marked with the real length it represents. Base the scale on a measurement, so the reader can work out the specimen's size.
Focus on low power first, mount your slide carefully, then work out and record the magnification of what you draw.
The three readings were written for this dataset using microscope facts studied in wikiHow's Focus a Microscope and Prepare Microscope Slides and OpenStax Microbiology, 2.3 Instruments of Microscopy. The five microscope flashcards are retained from the hoofer-content facts import.
More to read
Finding a clear microscope view safely
Start a school microscope session with a clear space on a steady table. Good room lighting lets you handle the glass safely. Switch on the lamp, or position the mirror on a model that uses one, so light reaches the specimen from below.
Choose the objective marked with the smallest magnification. On many school microscopes this is the shortest objective. Rotate the lens holder until that objective clicks into its working position. Secure the slide with the stage clips and centre the specimen over the opening.
Keep watching the gap between the objective and slide as you bring them closer. Stop before they touch. Looking through the eyepiece, turn the coarse control slowly so the gap increases. Once the specimen comes into view, change to the fine control for a sharper image. Focus controls move either the stage or the lens assembly, depending on the microscope.
After centring the detail you want, select the next stronger objective. Use fine focus at high power. Coarse movements can drive the objective into the glass and damage both.
The lenses enlarge cells and other tiny specimens that you cannot see clearly by eye. Follow your teacher's directions for the microscope you are using.
Choosing and making a microscope slide mount
Choose the mount to suit the sample. A dry mount contains no added liquid. A wet mount uses a little water to keep a moist or living specimen from drying during observation.
Check a clean slide against the light. Dirt and fingerprints can spoil the view. For a dry mount, put a thin sample, such as a hair or a pollen grain, near the centre and cover it gently. The sample needs to be thin enough for light to pass through.
For onion skin, make a wet mount. Add one water drop, then spread a small piece of the skin in it without folding it. Water containing tiny pond organisms can also provide a wet specimen.
Lower the cover glass in two stages: let one edge meet the liquid, then bring the other edge down gradually. Air can leave ahead of the moving edge. This reduces bubbles, which may appear as dark rings and block your view. Do not press on the glass.
A corner of paper towel can absorb water escaping around the edge. Use your teacher's help with glass and specimen preparation.
Microscope parts and two kinds of magnification
On a school compound microscope, the specimen rests on a slide secured to a platform called the stage. A lamp below it sends light through the specimen towards the objective and eyepiece.
You look into the eyepiece. Its magnification is often 10 times. Objectives are fitted to a rotating holder; common school values are 4, 10 and 40 times. Changing the objective changes the enlargement. Read the markings on your own lenses rather than assuming these values.
Coarse focus changes the distance between slide and objective by a large amount. Fine focus makes much smaller changes to sharpen the view. Use coarse focus for finding a specimen at low power and fine focus for work at high power.
To calculate total lens magnification, multiply the two lens values. An eyepiece of 10 times paired with an objective of 40 times gives 400 times. This describes enlargement of length, not the number of cell details visible. A drawing or printed photograph has its own magnification, found by dividing image length by actual length in matching units.
Light microscopy can show living cells. Electron microscopy can reveal smaller structures at much higher magnification, but standard electron microscopes cannot examine a living specimen.
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