1.1.1Cell Level Systems

Light Microscopes

5 min read 9 parts Lesson 2 of 13 1 quiz

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Section 1 of 9Why we need microscopes

Plant Cell — Light vs Electron Microscope

Plant cell under light microscope vs electron microscope

Examples of Microscope Specimen Stains — Simulation

Most cells are far too small to see with the naked eye.

A typical animal cell is about 0.01 to 0.03 mm across and a bacterium is only about 0.001 mm (1 micrometre).

Light microscope

The light microscope was the first instrument to reveal cells, and it is still widely used in schools and laboratories because it is cheap, portable and can show living specimens.

It works by passing light through a thin specimen and using glass lenses to magnify the image.

Section 2 of 9Parts and how to use one

1Eyepiece lens

Eyepiece lens the lens you look through, commonly ×10.

2Objective lenses

Objective lenses a rotating nosepiece holds lenses such as ×4, ×10 and ×40.

3Stage

Stage with clips to hold the slide in place.

4Light source

(a mirror or lamp) that shines light up through the specimen.

5Coarse focus knob

Coarse focus knob to move the stage quickly and fine focus knob to sharpen the image.

To view a specimen

place the slide on the stage;
- select the lowest-power objective;
- look from the side while turning the coarse focus so the lens is close to, but not touching, the slide;
- then look through the eyepiece and turn the coarse focus away from the slide until the specimen is roughly in focus;
- sharpen with the fine focus;
- and only then switch to a higher-power objective and refocus with the fine knob alone.
Starting at low power gives a wide field of view, so the specimen is easier to find, and it avoids the lens hitting the slide.

Section 3 of 9Preparing a slide

Preparing a Cheek-Cell Slide — Real Laser + Universal Wheel V11

Preparing and staining an onion epidermal slide

Parts of a light microscope used to examine prepared slides

Specimens must be thin enough for light to pass through. A common method for onion epidermis is:

Peel a very thin layer of epidermis from the inside of an onion scale leaf using forceps.

Place it flat on a drop of water on a slide.

Add a drop of a stain, such as iodine solution, which colours the nucleus and other structures so they stand out.

Lower a cover slip at an angle with a mounted needle to avoid trapping air bubbles, which would look like dark rings and block the view.

Blot excess liquid.

Other stains include methylene blue for animal cells such as cheek cells. Without a stain, cells are mostly transparent and low in contrast.

Section 4 of 9Magnification and resolution

Plant cells viewed with a light microscope

Plant cells viewed with a transmission electron microscope

Magnification is how many times larger an image is than the real object. For a light microscope the total magnification is the product of the lenses:

Total magnification = eyepiece magnification × objective magnification

For a ×10 eyepiece and a ×40 objective, total magnification = ×400. The relationship between size and magnification is given by:

Magnification = image size / actual size

You can rearrange it as actual size = image size / magnification.

Make sure the units match1 mm = 1000 micrometres (µm).
So an image of a cell measured as 20 mm long under ×400 magnification has an actual length of 20 / 400 = 0.05 mm, or 50 µm.

Resolution is the ability to distinguish two points that are close together as separate. Light microscopes can resolve objects down to about 0.2 micrometres, which limits them as light waves are too long to resolve smaller detail. Increasing magnification beyond this limit just gives a bigger, blurrier image.

Section 5 of 9Drawing and interpreting

Plant cell labelled (chloroplast, vacuole, cell wall)

Scientific drawings should be made in pencil with clear, continuous lines, with no shading or colouring, and should fill a good proportion of the space.

Drawing and interpreting

Draw only what you see, not what you expect to see.

Label structures using straight, ruled lines that do not cross, and include a title and the magnification or a scale bar.

With a light microscope you can see the nucleus, cytoplasm, cell membrane (although it appears as a boundary), cell wall, chloroplasts and large vacuoles in plant cells, but not ribosomes or the details of mitochondria, which need an electron microscope.

Section 6 of 9Using a graticule and the scale of cells

An eyepiece graticule is a small ruler inside the eyepiece.

Using a graticule and the scale of cells

It is calibrated for each objective by comparing it with a stage micrometer (a slide with a ruler of known size).

If 50 graticule divisions line up with 1 mm on the stage micrometer, one division equals 20 micrometres, so a cell that spans 3 divisions is 60 micrometres long.

This avoids having to measure a drawing.

Section 7 of 9Exam-style questions

A student looks at a cell using a ×10 eyepiece and a ×40 objective lens. The image of the cell is 12 mm long. Calculate the actual length of the cell in micrometres. [4]4 marks
Explain why a student should always start with the lowest-power objective lens when finding a specimen. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Magnification	How many times larger an image is than the actual object.
Resolution	The ability to distinguish two close points as separate.
Eyepiece lens	The lens at the top of a microscope that the observer looks through.
Objective lens	One of the lenses near the specimen on the rotating nosepiece.
Stain	A dye used to make structures in a specimen show up.
Cover slip	A thin glass or plastic sheet placed over a specimen on a slide.
Micrometre	One millionth of a metre; 1000 micrometres equal 1 mm.

Section 9 of 9Exam technique

Mixing units in a magnification calculation. Convert mm to micrometres before dividing, or state the unit used.

Using the coarse focus at high power, which can crash the lens into the slide.

Confusing magnification with resolution. A higher magnification does not always show more detail.

Drawing with shading, sketchy lines or colouring in a scientific drawing.

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.