Core practical: using a microscopeSpec 1.6
In this core practical you investigate biological specimens with a light microscope, calculate magnification and make a labelled scientific drawing. A common specimen is onion epidermis, which is a thin, single layer of plant cells.
Method
- Put a drop of water or stain (for example iodine solution) on a clean slide.
- Peel a thin layer of epidermis from the onion and lay it flat on the drop with forceps.
- Lower a coverslip on at an angle using a mounted needle, so air bubbles are not trapped.
- Clip the slide on the stage. Select the lowest power objective lens and use the coarse focus to bring the cells into focus, then the fine focus to make them sharp.
- Move to a higher power objective lens and focus again with the fine focus only.
- Draw what you see and label it.
Scientific drawings
- use a sharp pencil and make clear, continuous lines, with no shading or colouring
- draw large, filling most of the space, and only draw what you can see
- label with straight lines drawn with a ruler that touch the structure they name, and do not cross the labelling lines
- give the drawing a title and write the magnification
Total magnification of a microscope is the eyepiece lens magnification × the objective lens magnification. For a drawing, measure the image with a ruler and use the equation above. Rearranged: size of real object = size of image ÷ magnification.
Finding the real size
A student draws a cell. The drawing is 54 mm wide and the magnification is ×300. Calculate the real width of the cell in µm.
- real size = image size ÷ magnification
- 54 ÷ 300 = 0.18 mm
- 0.18 mm × 1000 = 180 µm
Answer: 180 µm
Finding the magnification
A cell is 90 µm long in real life. In a drawing it is 45 mm long. Calculate the magnification.
- Convert to the same unit: 45 mm × 1000 = 45 000 µm
- magnification = image size ÷ real size
- 45 000 ÷ 90 = 500
Answer: ×500
Quick check
Why can electron microscopes show more detail than light microscopes?
Show answer
They have a higher magnification and higher resolution.
How many nanometres are there in 1 µm?
Show answer
1000 nm.
Convert 0.05 mm into µm.
Show answer
50 µm (0.05 × 1000).
State the equation linking magnification, image size and real size.
Show answer
magnification = size of image ÷ size of real object.
Why is a coverslip lowered at an angle?
Show answer
To avoid trapping air bubbles.