Microscopes and magnification – GCSE Combined Science Biology

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Cells are far too small to see with your eyes alone. A microscope makes them big enough to study. Without microscopes, we would not know about nuclei, mitochondria or bacteria.

This matters because a lot of biology depends on seeing tiny structures and measuring them. Calculating size from a diagram is a skill that comes up again and again.

By the end you will be able to compare light and electron microscopes. You will describe how to make a slide. You will calculate magnification and real size, and change between units.

Light microscopes

Magnification means how many times bigger something looks. A light microscope shines light through a thin sample and uses lenses to magnify it. It magnifies up to about 1500 times.

It can show the nucleus, the cytoplasm and the cell wall. But its resolution is limited. Resolution is the smallest distance between two points that can still be seen as separate.

Think of zooming in on a phone photo. It gets bigger, but it can go blurry. A light microscope cannot show tiny structures such as ribosomes.

Electron microscopes

An electron microscope uses a beam of electrons instead of light. It has a much higher magnification and a much higher resolution.

It shows the inside of mitochondria and chloroplasts, and also ribosomes. Think of a sharp, high-quality photo that stays clear when you zoom in.

Electron microscopes have helped scientists understand cells in far more detail. But they are large and expensive. The sample is placed in a vacuum, so it cannot be alive.

Preparing a slide

To look at plant cells, peel a thin layer from an onion. Lay it on a drop of water on a slide. Add a drop of iodine solution as a stain. A stain is a dye that makes parts easier to see, such as the nucleus and cell wall.

Lower a coverslip gently at an angle, so no air bubbles get trapped. Think of putting a screen protector on a phone.

Start with the lowest-power objective lens. Focus with the coarse knob. Then switch to a higher power and focus with the fine knob.

The magnification equation

Use this equation. Magnification = image size ÷ real size. The image is what you see or draw. The real size is the true size of the object.

Both sizes must be in the same units before you divide. A magnification has no unit. We write it like ×200.

Say a 2 mm object looks 10 mm long in a drawing. The magnification is 10 ÷ 2 = ×5. You can rearrange it. Real size = image size ÷ magnification. Image size = real size × magnification.

Units and standard form

Cells are tiny, so we use small units. 1 millimetre (mm) = 1000 micrometres (µm). 1 micrometre = 1000 nanometres (nm).

To change mm to µm, multiply by 1000. To change µm to mm, divide by 1000. Think of changing metres to millimetres. A smaller unit means a bigger number.

Standard form is a way to write very small or very large numbers using powers of ten. 0.002 mm is 2 × 10⁻³ mm. That is 2 µm.

It is useful to estimate the size of a cell. Compare it with something of known size, such as the scale bar. A typical animal cell is of the order of 10 µm to 100 µm across, so its size is about 10⁻⁵ m. A bacterium is about 1 µm to 5 µm long. Use an estimate to check your calculated answer is a sensible size.

Using the equation in an exam

First, convert both sizes to the same unit. A drawing 36 mm long shows a cell that is 12 µm long. Change 36 mm to 36 000 µm. Then 36 000 ÷ 12 = ×3000.

If you forget to convert, your answer will be wrong by a factor of 1000.

A scale bar shows a real length on the drawing. Measure the bar and the cell with a ruler. Then use real size = image size ÷ magnification.

Recap

Light microscopes are cheaper and can show living cells. Electron microscopes have higher magnification and resolution.

Magnification = image size ÷ real size. Convert to the same unit first.

1 mm = 1000 µm and 1 µm = 1000 nm.

Magnification has no unit. Write it like ×200.