Future Scientists

Revision note 1.5

Microscopes and Biological Drawing

Light microscopes allow us to observe cells directly. Reliable microscopy depends on careful slide preparation, systematic focusing and an accurate scientific record.

Required practicalCoreDiagram

What you need to know

  • Identify the main parts of a light microscope and state their functions.
  • Prepare and observe a temporary slide safely.
  • Focus a specimen from low to high power.
  • Produce a valid biological drawing with a scale.
  • Compare light and electron microscopes.

Parts of a light microscope

View through a light microscope showing rectangular onion epidermal cells with stained nuclei.
Onion epidermal cells seen through a light microscope. A stain makes the nuclei easier to distinguish.
PartFunction
Eyepiece lensMagnifies the image for the observer, commonly ×10.
Objective lensesProvide different magnifications, for example ×4, ×10 and ×40.
StageSupports the slide; clips hold it in place.
Light source or mirrorIlluminates the specimen from below.
Coarse-focus controlMoves the stage or lens by a large distance for rough focusing at low power.
Fine-focus controlMakes small adjustments to sharpen the image.
Total microscope magnification

Eyepiece magnification × objective-lens magnification. A ×10 eyepiece with a ×40 objective gives ×400 total magnification.

Preparing a temporary slide

1. Thin specimenPlace a thin section or cell sample on a clean slide.
2. Add liquidAdd water or a suitable stain.
3. Lower coverslipUse a mounted needle to lower it at an angle.
4. Remove excessBlot liquid carefully if needed.

Onion epidermis can be stained with iodine solution. Cheek cells can be stained with methylene blue. A stain increases contrast, making otherwise transparent structures easier to see.

Why lower the coverslip at an angle?

This reduces the chance of trapping air bubbles. Bubbles can obscure cells or be mistaken for structures.

Wear eye protection when using stains, handle glass slides carefully and use a sterile cotton bud for cheek cells. A cheek-cell sample should be taken gently from the inside of the student's own mouth and the cotton bud disposed of safely.

Focusing the specimen

  1. Rotate the lowest-power objective into place.
  2. Place the slide on the stage and centre the specimen over the light.
  3. While looking from the side, bring the objective close to the slide without touching it.
  4. Look through the eyepiece and use coarse focus to bring the specimen into view.
  5. Use fine focus to sharpen the image and adjust the light if necessary.
  6. Centre the area of interest before moving to a higher-power objective.
  7. At high power, use fine focus only.
Misconception AlertStart with the highest-power objective so the specimen is easier to find.Select to reveal the correctionSelect to hide the correction
Correct understanding

Begin with the lowest-power objective. Its wider field of view makes the specimen easier to locate and focus.

As magnification increases, the field of view becomes smaller, the image often becomes dimmer and fine focusing becomes more important.

Producing a biological drawing

  • Use a sharp pencil and clear, single lines.
  • Make the drawing large enough to show detail.
  • Draw what you observe, not an idealised textbook cell.
  • Do not shade or sketch.
  • Use straight label lines drawn with a ruler.
  • Do not let label lines cross.
  • Give the drawing a descriptive title.
  • Include magnification or, preferably, a scale bar.
Exam tip

If asked to draw a group of cells, show how the cells join and preserve the relative size and proportions of the structures. A scientific drawing is evidence, not decoration.

Light and electron microscopes

Magnification tells you how many times larger the image is than the object. Resolution is the ability to distinguish two close points as separate. Resolution determines how much useful detail can be seen.

FeatureLight microscopeElectron microscope
RadiationVisible lightBeam of electrons
MagnificationLowerMuch higher
ResolutionLowerMuch higher
SpecimensCan observe living specimensSpecimens are dead and specially prepared
DetailCells and some large organellesMuch smaller structures and internal detail

Electron microscopes increased biological understanding by revealing finer subcellular structures that light microscopes could not resolve.

Quick retrieval check

1. Why should focusing begin with the lowest-power objective?
It gives a wider field of view, making the specimen easier to find and centre.
2. Why is a stain used?
To increase contrast so transparent cell structures are easier to see.
3. State three rules for biological drawings.
Any three from: sharp pencil, single clear lines, no shading, large drawing, straight non-crossing label lines, descriptive title, scale or magnification.
4. What is the difference between magnification and resolution?
Magnification is how many times larger the image is than the object. Resolution is the ability to distinguish two close points as separate.

Exam connection

Question

A student can make a cell image larger on a computer screen, but no additional detail becomes visible. Explain why.

Show the mark points
  • The displayed image has greater magnification.
  • The resolution of the original image has not increased.
  • No additional information was captured, so close structures still cannot be distinguished.