Cells, microscopes and creepy crawlies

And all the other things you can’t see!

How small are they really?

E. coli: 2-3 μm

Influenza virus: 0.130 μm OR 130 nm

Red Blood cell: 8 μm

Onion cell: 250-400 μm

Let’s have a closer look!!!

1000 μm!!!

When you look in a microscope the circle of light that you see is called the ‘field of view’

Field of view

But how do we know how small a cell/specimen really is?It depends on the magnificationAs you increase the magnification the

field of view decreases

Measuring a cell!

Power Objective Lens magnification

Ocular Lens magnification

Total magnification

Field diameter (mm)

Field diameter (μm)

Low 4 10 40 4.0 4,000Medium

10 10 100 1.6 1,600

High 40 10 400 0.4 400

1000 μm!!!

1,600 μm

1,600 μm

Field diameter ÷ no. cells

≈ 533 μm ÷ 3

Measuring a cell!

Power Objective Lens magnification

Ocular Lens magnification

Total magnification

Field diameter (mm)

Field diameter (μm)

Low 4 10 40 4.0 4,000Medium

10 10 100 1.6 1,600

High 40 10 400 0.4 400

4,000 μm Low

Medium

High

4,000 μm ÷ 2

≈ 2,000 μm1,600 μm

1,600 μm ÷ 2

≈ 800 μm

400 μm

400 μm ÷ 2 ≈ 200 μm

Field diameter ÷ no. cells

We’re going to look at a few specimenDraw what you see Write down the magnification

Low = 4 × 10, Medium = 10 × 10, High = 40 × 10Guess how many of them you could fit side by

sideUse the formula to work out how long / wide

your specimen are!

Then we’ll come back to our tables to discuss what we found

Measuring your own specimens!