Parts of a microscope: a labelled guide

A labelled microscope diagram shows the parts of a light microscope: the eyepiece lens you look through, the objective lenses that magnify the specimen, the stage where the slide sits, the coarse and fine focus knobs, and the light source underneath. The total magnification is the eyepiece magnification multiplied by the objective magnification, so a ×10 eyepiece with a ×40 objective gives ×400.

Labelled microscope: the parts and their jobs

These are the parts of a school light microscope, from top to bottom, with what each one does. In an exam diagram, label lines should touch the part exactly and not cross each other.

Light passes up from the light source, through the thin specimen on the slide, into the objective lens, and then up through the eyepiece lens to your eye. That is why specimens must be thin: light has to get through them.

To see the parts in action, try the virtual microscope: focus on onion cells, blood, a leaf section, pond water and more, switch objectives and measure cells with a scale bar.

How to use a light microscope

AQA's required practical activity 1 for GCSE Biology is to use a light microscope to observe, draw and label a selection of plant and animal cells, with a magnification scale. The usual method:

1. Clip the prepared slide onto the stage, with the specimen over the hole.
2. Turn the nosepiece to the lowest power objective lens, usually ×4.
3. Looking from the side, turn the coarse focus to bring the lens and slide close together, without touching.
4. Look through the eyepiece and turn the coarse focus the other way, moving them apart, until the image comes into focus.
5. Use the fine focus to make the image sharp.
6. To magnify more, turn to a higher power objective and refocus using the fine focus only.
7. Draw what you see with a sharp pencil, using clear unbroken lines and no shading, and label it. Add the magnification or a scale bar.

Calculating magnification

Total magnification of the microscope

A ×10 eyepiece with a ×40 objective gives 10 × 40 = ×400. With the ×4 objective it would be ×40, and with the ×10 objective, ×100.

The magnification equation

To link the size of an image, such as a drawing or photo, with the real size of the object, AQA's specification gives:

Both sizes must be in the same units before you divide. Remember 1 mm = 1,000 micrometres (µm) and 1 µm = 1,000 nanometres (nm). AQA expects you to use the prefixes centi, milli, micro and nano, and to give answers in standard form where it makes sense.

Example 1: finding the real size

A cell in a photo is 20 mm wide at a magnification of ×400. Rearrange the equation: real size = image size ÷ magnification = 20 ÷ 400 = 0.05 mm. In micrometres, 0.05 × 1,000 = 50 µm, which is 5 × 10⁻⁵ m in standard form.

Example 2: finding the magnification

A cell is really 0.08 mm long, and your drawing of it is 32 mm long. Magnification = 32 ÷ 0.08 = ×400. Magnification has no units, because the units cancel.

The magnification calculator rearranges the equation for any of the three values and converts the units for you.

Light microscope vs electron microscope

Resolution is the ability to tell two points that are close together apart. Making an image bigger without better resolution just gives a bigger blur. AQA's specification says an electron microscope has much higher magnification and resolving power than a light microscope, which has let biologists see and understand many more sub-cellular structures.

Common mistakes

• Adding the lens magnifications. Multiply them.
• Mixing units in the magnification equation. Convert both sizes to mm or both to µm first.
• Using coarse focus on high power. Only use the fine focus once you are on a high power objective.
• Shading or sketchy lines in a biological drawing. Use single, clear lines.
• Saying electron microscopes are better because they magnify more. The key reason is higher resolution.

To practise cell biology in a short timed paper, try today's GCSE Biology Foundation or GCSE Biology Higher paper. For the cells you will see down the microscope, read specialised cells.

Read next

Biology · CellsVirtual microscope: focus, magnify and measure cellsFree virtual microscope: focus on onion cells, blood, a leaf and live pond water, switch objectives from ×4 to ×100 and measure cells with a scale bar.