The microscope in cell studies
Seeing the invisible
- Cells are far too small to see with your eyes — you need a microscope.
- A light microscope shines light through a thin specimen; an electron microscope sees far more detail.
- Two ideas you must keep separate: magnification and resolution.
Light and electron microscopes
- A light microscope shines light through a thin specimen on a slide (with a stain to show parts, and a cover slip on top).
- An electron microscope uses beams of electrons, which have a much shorter wavelength — so its resolution is far higher and it shows tiny structures.
- Two kinds: scanning (SEM, 3-D surface) and transmission (TEM, inside detail through a thin slice).
Practice
Why does an electron microscope have a much higher resolution than a light microscope?
Shorter wavelength → higher resolution, so finer detail can be seen.
Magnification and actual size
$$\text{magnification} = \frac{\text{size of image}}{\text{actual size}} \qquad \text{actual size} = \frac{\text{size of image}}{\text{magnification}}$$
- The top and bottom must use the same unit. Cells are tiny: $1\ \text{mm} = 1000\ \mu\text{m}$, $1\ \mu\text{m} = 1000\ \text{nm}$.
- Worked: at $\times 5000$, a chloroplast measures $25\ \text{mm}$ → actual size $= 25 \div 5000 = 0.005\ \text{mm} = 5\ \mu\text{m}$.
Practice
Which equation gives magnification?
Magnification = size of image ÷ actual size (no units).
Practice
How many micrometres (µm) are in 1 mm?
1 mm = 1000 µm (and 1 µm = 1000 nm).
Practice
At magnification ×5000, a structure measures 25 mm in the image. What is its actual size, in µm?
25 mm ÷ 5000 = 0.005 mm = 5 µm.
Resolution vs magnification
- Magnification = how many times bigger the image is.
- Resolution = the smallest distance between two points that still lets you see them as two separate points.
- Making an image bigger does not always show more detail — past a point you just get a bigger blur. Resolution sets the real limit.
Practice
Resolution is:
Resolution is the detail limit; magnification alone cannot show detail finer than the resolution.
You've got it
Key idea
- light microscope (light, lower resolution) vs electron microscope (electrons, much higher resolution)
- $\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$ — same units top and bottom ($1\ \text{mm}=1000\ \mu\text{m}$)
- resolution = smallest distance to see two points apart — it sets the real detail limit
- magnifying more doesn't add detail beyond the resolution