BIOLOGY · Stage 7 · Chapter 2: Cells and Organisation
Biology 2.4 Using a microscope — Specimen Preparation and Magnification
5 min read 7 parts Lesson 4 of 10
Lesson summary
2.4 Using a microscope — Specimen Preparation and Magnification — Alhussan International Academy — Biology (Stages 7–9) notes: Prepare a slide, use a light microscope correctly and calculate magnification and actual size.PartFunctionEyepiece lensIs looked through and magnifies th…
- Light path
- Why lower slowly
- Cheek cells
- Many cells are colourless
- Mitochondria
- Total magnification
Section 1 of 7Parts of a light microscope
| Part | Function |
|---|---|
| Eyepiece lens | Is looked through and magnifies the image |
| Objective lens | Makes the specimen look bigger |
| Stage | Holds the slide, with clips to keep it still |
| Coarse focus | Moves the stage or lens a large distance to bring the image into focus |
| Fine focus | Makes the image sharp |
| Light source or mirror | Provides light, and some microscopes have a mirror that is moved to get enough light |
| Diaphragm | Controls the brightness |
- Light path
- light passes up through the thin specimen and then through the objective lens and the eyepiece lens.
- Specimen
- it must be thin enough for light to pass through.
Section 2 of 7Using a microscope
- Move the stage of the microscope to its lowest position.
- Put the object on the stage and keep it in place using the clips.
- Choose the objective lens with the lowest magnification and move it into position.
- Look through the eyepiece and turn the coarse focus knob slowly until the object comes into focus.
- Turn the fine focus knob slightly to make the image sharper.
- Draw the observations in pencil and record the magnification.
- Repeat the steps with an objective lens of higher magnification to see more detail, using only the fine focus.
Section 3 of 7Preparing a slide
- Place a thin piece of tissue, for example the transparent inner skin of an onion, on a clean glass slide.
- Add a drop of water or a drop of stain such as iodine solution, which makes parts such as the nucleus easier to see.
- Lower a coverslip slowly at an angle using a mounted needle.
- Blot away extra liquid with paper towel.
- Why lower slowly
- it prevents air bubbles, which look like dark rings and can be mistaken for cells.
- Cheek cells
- they can be prepared in the same way using a stain such as methylene blue.
Section 4 of 7Staining cells
- Many cells are colourless
- this makes them hard to see under the microscope.
- Stains = coloured dyes added to specimens to make cells easier to see.
- A stain on onion cells can make the cell walls pink and the nucleus dark purple.
- Mitochondria
- they cannot be seen in light microscope images because they are too small.
Section 5 of 7Total magnification
- Total magnification = eyepiece lens magnification × objective lens magnification.
- Both lenses magnify the specimen, so their powers are multiplied.
- Typical lenses
- an eyepiece of ×10 with objectives of ×4, ×10 and ×40.
- These give total magnifications of ×40, ×100 and ×400.
- Highest power
- the best light microscopes can magnify up to 1000 times.
Section 6 of 7Magnification, image size and actual size
1Magnification2 points
= size of image ÷ actual size of the object.
- Rearranged, actual size = image size ÷ magnification.
- Rearranged, image size = actual size × magnification.
2Units2 points
- all three quantities must use the same units.
- Small objects are usually given in micrometres, where 1 mm = 1000 µm.
Section 7 of 7Drawing and caring for the microscope
- Drawing
- use a sharp pencil, clear continuous lines, no shading or colouring, and a size big enough to show detail.
- Labels
- write labels in pencil with ruler-straight lines that never cross, and give the title and magnification.
- Blurred image
- the focus needs adjusting or the specimen is too thick.
- Dark image
- the diaphragm should be opened or the lamp made brighter.
- Bright circle without cells
- the view may be of a bubble or an empty part of the slide.
- Care
- carry the microscope with two hands, clean lenses with lens tissue and switch off the lamp after use.
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