Microscopy and Magnification

A light microscope (or optical microscope) uses visible light and glass lenses to magnify specimens.

Key Features

• Maximum magnification: approximately $\times 1500$
• Maximum resolution: approximately $200 \text{ nm}$ (0.2 μm)
• Can view living and dead specimens
• Specimens can be stained to make structures more visible
• Relatively cheap and portable

Parts of a Light Microscope

Total Magnification

$\text{Total magnification} = \text{Eyepiece magnification} \times \text{Objective magnification}$

Example: Eyepiece = ×10, Objective = ×40 $\text{Total magnification} = 10 \times 40 = \times 400$

Electron microscopes use a beam of electrons instead of light to form an image.

Types

Transmission Electron Microscope (TEM):

• Electrons pass through a thin specimen
• Produces 2D images of internal structures
• Very high magnification (up to $\times 1{,}000{,}000$) and resolution (~0.1 nm)

Scanning Electron Microscope (SEM):

• Electrons bounce off the surface of a specimen
• Produces 3D images of surface structures
• Lower resolution than TEM but shows surface detail

Comparing Light and Electron Microscopes

Resolution vs Magnification

• Magnification is how much larger the image appears compared to the real object
• Resolution is the ability to distinguish between two points that are very close together
• A microscope can magnify an image greatly, but if the resolution is poor, the image will be blurry — you can't see more detail by increasing magnification beyond the resolution limit

The key formula you must know:

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$

Or written as the triangle:

$M = \frac{I}{A}$

This can be rearranged to find:

$\text{Image size} = \text{Magnification} \times \text{Actual size}$ $\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}}$

Unit Conversions

You must ensure both measurements are in the same units before calculating:

Very small measurements are often written in standard form:

$5 \text{ μm} = 5 \times 10^{-6} \text{ m}$ $200 \text{ nm} = 200 \times 10^{-9} \text{ m} = 2 \times 10^{-7} \text{ m}$

You may be examined on how to prepare and observe slides.

Preparing a Slide (e.g., Onion Epidermal Cells)

1. Place a thin layer of onion epidermis on a clean glass slide
2. Add a drop of iodine stain (to make the nucleus and cell wall visible)
3. Carefully lower a coverslip at an angle to avoid trapping air bubbles
4. Place the slide on the microscope stage

Viewing the Specimen

1. Start with the lowest power objective lens (e.g., ×4)
2. Use the coarse focus to get a rough image
3. Switch to a higher power objective lens (e.g., ×10, then ×40)
4. Use the fine focus to sharpen the image
5. Draw what you see — use clear, continuous lines with labels

Drawing Rules for Biological Diagrams

• Use a sharp pencil and clear, continuous lines (no shading or colouring)
• Draw what you actually see, not what you think you should see
• Include a title and state the magnification
• Use label lines (not arrows) drawn with a ruler, touching the structure
• The drawing should be large and fill at least half the space provided

Question: A cell is observed at ×400 magnification. The image of the cell is 12 mm across. What is the actual size of the cell in μm?

Solution:

$\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} = \frac{12 \text{ mm}}{400} = 0.03 \text{ mm}$

Convert to micrometres: $0.03 \text{ mm} \times 1000 = 30 \text{ μm}$

The actual cell is 30 μm across.

Question: A bacterium is 2 μm long. A student draws it 40 mm long. What magnification was used?

Solution:

First, convert to the same units. Convert 2 μm to mm: $2 \text{ μm} = 0.002 \text{ mm}$

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} = \frac{40 \text{ mm}}{0.002 \text{ mm}} = \times 20{,}000$

Question: An organelle has an actual size of 500 nm. It is viewed at ×20,000 magnification. What is the image size in mm?

Solution:

Convert 500 nm to mm: $500 \text{ nm} = 0.0005 \text{ mm}$

$\text{Image size} = \text{Magnification} \times \text{Actual size} = 20{,}000 \times 0.0005 = 10 \text{ mm}$

• Always show your working in magnification calculations — even if you get the wrong answer, you can gain method marks.
• Convert units first — the most common error is using mm for image size and μm for actual size without converting.
• When asked about advantages of electron microscopes, focus on higher resolution (more detail), not just higher magnification.
• In required practical questions, describe staining, coverslip technique, and focusing progression (low → high power).

• Light microscopes use light and lenses; max magnification ~×1500, resolution ~200 nm.
• Electron microscopes (TEM and SEM) use electrons; max magnification ~×1,000,000, resolution ~0.1 nm.
• The magnification formula: $M = \frac{I}{A}$ — always check units are the same.
• Resolution limits the useful detail you can see — more magnification beyond the resolution limit just gives a bigger, blurrier image.
• Required practical: prepare slides carefully, start at low power, use staining for contrast.