🔬 Light microscope
Lower magnification.
Lower resolving power, so fine detail is blurred.
Shows cells and large structures such as the nucleus.
Cell Structure » Microscopes and Magnification
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.1.5
- How microscopes have developed over time
- How light and electron microscopes differ in magnification and resolution
- How electron microscopes helped us to see sub-cellular structures
- How to calculate magnification, image size and real size
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Unlock This CourseWhy we need microscopes
Most cells are far too small to see with the naked eye. To study them, biologists use microscopes. A microscope makes a small object look bigger and shows detail that we could never see on our own.
The kind of microscope we use decides how much detail we can see. Two ideas matter here: how much bigger the image is, and how clear the detail is.
Key terms:
- Magnification: how many times bigger the image looks than the real object.
- Resolution (resolving power): the ability of a microscope to show two points that are close together as two separate points. Better resolution means finer detail.
- Light microscope: a microscope that uses light and glass lenses to magnify a specimen.
- Electron microscope: a microscope that uses a beam of electrons instead of light to form a magnified image.
How microscopes have developed
Microscopy has changed a lot over time, because new technology lets us see more.
- In the 1600s, scientists began to build light microscopes with glass lenses. In 1665 Robert Hooke looked at thin slices of cork and described tiny box-like spaces that he called cells.
- Around the same time, Antonie van Leeuwenhoek made very good single-lens microscopes. He was one of the first people to see tiny living things such as bacteria.
- Over the next few centuries, lenses got better, so light microscopes gave clearer and brighter images.
- In the 1930s, the first electron microscopes were built. They could show much more than any light microscope.
Each new type of microscope let biologists see things that were hidden before. That is why our ideas about cells keep growing.
Light microscopes and electron microscopes
A light microscope is cheap, small and easy to use. It can show living cells, and it can show some structures in them such as the nucleus. Its limit is that light cannot show very tiny detail.
An electron microscope uses a beam of electrons. It has a much higher magnification and a much higher resolving power than a light microscope. This means it can be used to study cells in much finer detail.
⚡ Electron microscope
Much higher magnification.
Much higher resolving power, so fine detail is clear.
Shows very small structures inside cells.
Magnification is not the same as resolution
Magnification makes the image bigger. Resolution makes the image clearer. If you enlarge a blurry photo, it is bigger but it is still blurry. An electron microscope gives both: a bigger image and much more detail.
What electron microscopes showed us
With a light microscope, biologists could see the nucleus, the cytoplasm and the overall shape of a cell. The very small parts inside the cell were too tiny to see clearly.
Electron microscopes changed this. They let biologists see the fine detail of structures such as mitochondria, chloroplasts and the cell membrane, and see ribosomes and the plasmids in bacteria, which a light microscope cannot show. Once they could see these parts, they could work out how they are built and how they do their jobs. So electron microscopy has helped us to understand many more sub-cellular structures than we knew about before.
Calculating magnification
You need to be able to work with three quantities: the size of the image, the size of the real object and the magnification. They are linked by one formula.
The magnification formula
magnification = size of image ÷ size of real object
The magnification has no units. It is written with a times sign, for example ×400.
You can rearrange the formula to find the other quantities:
- size of image = magnification × size of real object
- size of real object = size of image ÷ magnification
Key rule: the image size and the real size must be in the same units before you divide. Always convert first.
Units you must know
- 1 millimetre (mm) = 1000 micrometres (µm)
- 1 micrometre (µm) = 1000 nanometres (nm)
- 1 millimetre (mm) = 1 000 000 nanometres (nm)
To go from a bigger unit to a smaller one, multiply by 1000 each step (mm to µm, then µm to nm). To go from a smaller unit to a bigger one, divide by 1000 each step.
Remember from the lesson Eukaryotes and Prokaryotes that very large and very small numbers are written in standard form. You may be asked to give your answer in standard form if that is sensible.
Worked example 1: finding the magnification
A photograph of a mitochondrion taken with an electron microscope is 30 mm long. The real mitochondrion is 2 µm long.
Step 1: convert to the same unit. 30 mm × 1000 = 30 000 µm.
Step 2: magnification = 30 000 ÷ 2 = 15 000.
Answer: ×15 000, which is 1.5 × 104 in standard form.
Worked example 2: finding the real size
A cell is viewed under a light microscope. The image of the cell is 56 mm long and the magnification is ×400.
real size = image size ÷ magnification = 56 ÷ 400 = 0.14 mm.
In micrometres: 0.14 × 1000 = 140 µm.
Worked example 3: finding the image size
A small structure has a real length of 0.05 mm. A drawing is made with a magnification of ×200.
image size = magnification × real size = 200 × 0.05 = 10 mm.
Worked example 4: standard form
An image is 20 mm long and the real object is 100 nm long.
20 mm = 20 000 µm = 20 000 000 nm. Magnification = 20 000 000 ÷ 100 = 200 000 = 2 × 105.
The real size in micrometres: 100 nm ÷ 1000 = 0.1 µm = 1 × 10-1 µm.
Common mistakes
Dividing before converting units, so the answer is out by a factor of 1000. Always change both sizes to the same unit first.
Putting the formula upside down. The image is bigger than the real object, so the magnification of a microscope should be more than 1.
Mixing up magnification and resolution. Magnification is how much bigger. Resolution is how much detail.
Writing units after the magnification. Magnification is just a number, such as ×400.
Exam-style question
(a) State two ways in which an electron microscope is better than a light microscope. [2 marks]
(b) A scientist views a cell with an electron microscope. The image is 24 mm long. The real cell is 8 µm long. Calculate the magnification. [3 marks]
(c) Explain how electron microscopes have increased our understanding of cells. [2 marks]
Model answer
(a) It has a higher magnification (1). It has a higher resolving power, so it shows finer detail (1).
(b) 24 mm = 24 000 µm (1). Magnification = 24 000 ÷ 8 (1) = ×3000 (1).
(c) They let us see sub-cellular structures in much finer detail (1), so biologists could understand more about their structure and function (1).
Exam tip
In part (b), the units are different (mm and µm). Write the conversion on its own line so you pick up the method mark, even if the final number is wrong.