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Biology · Cell Structure

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Microscopy and Magnification

Most cells are too small to see without help. This lesson covers how light and electron microscopes work, how to use a light microscope, and how to calculate magnification and real size.

  • 9 key terms
  • All boards

Learning Objectives

  1. 1Describe how a light microscope is used to observe plant and animal cells.
  2. 2Compare light microscopes with electron microscopes in terms of magnification and resolution.
  3. 3Calculate magnification, image size and real size using the magnification formula.
  4. 4Explain how electron microscopy has increased our understanding of sub-cellular structures.

Retrieval practice

  1. 1

    What is the function of the nucleus?

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    It contains the genetic material and controls the cell's activities.

  2. 2

    How many micrometres are there in 1 mm?

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    1000.

  3. 3

    Which structure in a cell is the site of aerobic respiration?

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    The mitochondria.

  4. 4

    Which cells have a cell wall made of cellulose?

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    Plant cells.

Seeing the invisible

The human eye cannot see anything much smaller than about 0.1 mm (100 µm), and most cells are smaller than that. A microscope makes a specimen look bigger, which is called magnification, and also shows more detail, which is called resolution. These two ideas are different, and examiners test the difference often.

Two kinds of microscope

  • Light microscope

    Shines light through a thin specimen and uses glass lenses to magnify it. It is cheap, easy to use and can show living cells, but its detail is limited.

  • Electron microscope

    Fires a beam of electrons at the specimen. Electrons have a much shorter wavelength than light, so it can magnify much more and show much finer detail, but it is large, expensive and the specimen is dead.

  • Magnification

    How many times bigger the image is than the real object.

  • Resolution

    How clearly two points that are close together can be told apart. Also called resolving power.

Light microscope and electron microscope compared

Light microscope

  • Magnifies up to about ×1500
  • Resolution about 0.2 µm (200 nm)
  • Specimen can be alive, and colour stains can be used
  • Shows nuclei, cell walls and chloroplasts, but not much more

Electron microscope

  • Magnifies up to about ×2 000 000
  • Resolution about 0.2 nm, about 1000 times better
  • Specimen is dead and the image is black and white
  • Shows ribosomes, plasmids and the folded inner membranes of mitochondria

What each part does

You must be able to name the parts of a light microscope and say what each one is for.

  • Eyepiece lens

    You look through it. It magnifies the image, usually ×10.

  • Objective lenses

    Different strengths, commonly ×4, ×10 and ×40. You choose one by turning the nosepiece.

  • Stage and clips

    The stage holds the slide, and the clips keep it from moving.

  • Light source

    Lights the specimen from underneath so it can be seen.

  • Coarse and fine focus

    The coarse wheel moves the stage a long way to find the image. The fine wheel gives a sharp, clear picture.

Using a light microscope (required practical)

Always start on the lowest power so you can find the specimen.

  1. 1 Prepare the slide

    Put a thin piece of tissue, such as onion skin, on a slide, add a drop of stain such as iodine, and lower a coverslip on at an angle so no air bubbles are trapped.
  2. 2 Clip it on the stage

    Place the slide on the stage and secure it with the clips.
  3. 3 Choose the lowest power

    Turn the nosepiece to the ×4 objective lens, so that most of the slide is in view.
  4. 4 Focus

    Looking from the side, use the coarse focus wheel to raise the stage until the slide is close to the lens. Then look through the eyepiece and turn the wheel the other way until the image is roughly sharp.
  5. 5 Sharpen the image

    Use the fine focus wheel to make the picture clear. Move to a higher power only when you are happy, and refocus with the fine wheel.
  6. 6 Draw what you see

    Use a sharp pencil, clear single lines and no shading. Label the structures and write the magnification beside the drawing.

Calculating magnification

Magnification is how many times bigger the image is than the real object. Always get both sizes into the same units before you divide.

  • The formula

    \(\text{magnification} = \dfrac{\text{size of image}}{\text{size of real object}}\), which is \(M = I \div A\).

  • Total magnification

    Multiply the eyepiece lens by the objective lens: \(\times 10\) and \(\times 40\) gives \(\times 400\).

  • Units

    \(1\text{ mm} = 1000\ \mu\text{m}\) and \(1\ \mu\text{m} = 1000\text{ nm}\). Convert before you calculate.

  • Large and small numbers

    Answers are often given in standard form, for example \(2 \times 10^{5}\).

Calculating the magnification

A student draws a cell that is 36 mm wide in the drawing. The real cell is 12 µm wide. Calculate the magnification of the drawing.

Show the solutionHide the solution
  1. 1 Convert to the same units Image: 36 mm = 36 000 µm. Real size: 12 µm.
  2. 2 Write the formula \(M = \dfrac{I}{A}\)
  3. 3 Substitute and calculate \(M = \dfrac{36\,000}{12} = 3000\), so the drawing is \(\times 3000\).

AnswerThe magnification is ×3000.

Calculating the real size of a cell

A photograph of a cell shows it as 54 mm wide. The magnification is ×1500. Calculate the real width of the cell in micrometres.

Show the solutionHide the solution
  1. 1 Rearrange the formula \(A = \dfrac{I}{M}\)
  2. 2 Substitute \(A = \dfrac{54}{1500} = 0.036\text{ mm}\)
  3. 3 Convert the units \(0.036\text{ mm} \times 1000 = 36\ \mu\text{m}\)

AnswerThe real width is 36 µm.

Case study

How electron microscopes changed biology

Until the 1930s, scientists could study cells only with light microscopes, which cannot show anything smaller than about 0.2 µm. The first electron microscopes, built in the 1930s, could do much better. They revealed the ribosomes where proteins are made, the folded inner membranes of mitochondria, and the stacks inside chloroplasts. Knowing these structures in detail explained how respiration, photosynthesis and protein production really work.

1930s The first electron microscopes are built
0.2 nm The detail a modern electron microscope can resolve, about 1000 times finer than light

Do not mix them up

Magnification makes an image bigger. Resolution makes it clearer.

An image can be magnified a million times and still be blurred if the resolution is poor.

Microscopy

Microscopy

  • Light microscope

    • up to ×1500
    • living cells
    • cheap
    • resolution 0.2 µm

  • Electron microscope

    • up to ×2 000 000
    • dead specimens
    • resolution 0.2 nm

  • Magnification

    • image size ÷ real size

  • Resolution

    • clarity: separating two close points

  • Units

    • 1 mm = 1000 µm
    • 1 µm = 1000 nm

Summary and exam focus

  • A light microscope uses lenses and light and can show living cells up to about ×1500.
  • An electron microscope has much higher magnification and resolution, so it shows far finer detail.
  • Magnification equals image size divided by real size, with both in the same units.
  • Always begin on the lowest power and focus with the coarse wheel before the fine wheel.
  • Electron microscopes revealed ribosomes, plasmids and the inside of mitochondria.

Exam focus

Explain how electron microscopy has increased our understanding of sub-cellular structures. (4 marks) (4 marks)

Name a structure the light microscope cannot show, link it to the electron microscope's higher resolution, and say what scientists learned from it. Use the words magnification and resolution correctly.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Magnification
How many times bigger an image is than the real object.
Resolution
The ability to see two points that are close together as separate. Also called resolving power.
Light microscope
A microscope that uses light and glass lenses to magnify a specimen.
Electron microscope
A microscope that uses a beam of electrons, giving much higher magnification and resolution.
Eyepiece lens
The lens you look through, usually ×10.
Objective lens
The lens just above the specimen, giving different magnifications.
Stage
The platform on which the slide is placed.
Stain
A coloured chemical, such as iodine, that makes parts of a cell easier to see.
Specimen
The object being looked at under a microscope.

Questions and answers

16 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question State 1 mark Easier

State the formula used to calculate magnification.

Mark scheme — 1 mark available

  • Magnification = image size ÷ real size — 1 mark

Model answer

Magnification = size of image ÷ size of real object.

2. Exam question Give 2 marks Easier

Give two advantages of using an electron microscope instead of a light microscope.

Mark scheme — 2 marks available

  • Higher magnification — 1 mark
  • Higher resolution / shows more detail — 1 mark

Model answer

It has a higher magnification, and it has a higher resolution so more detail can be seen.

3. Exam question Calculate 3 marks Easier

Figure 1 shows a drawing of a cell. The magnification is ×1500. Calculate the real width of the cell. Give your answer in micrometres.

A drawing of a cell with its width marked as 54 mm and a magnification of times 1500.

Mark scheme — 3 marks available

  • Uses real size = image size ÷ magnification — 1 mark
  • 54 ÷ 1500 = 0.036 mm — 1 mark
  • Converts to 36 µm — 1 mark

Model answer

real size = 54 ÷ 1500 = 0.036 mm. 0.036 mm × 1000 = 36 µm.

4. Exam question Calculate 2 marks Easier

A student looks at a slide using a ×10 eyepiece lens and a ×40 objective lens. Calculate the total magnification, and name the wheel the student should use to make the image sharp.

Mark scheme — 2 marks available

  • 10 × 40 = ×400 — 1 mark
  • Fine focus wheel — 1 mark

Model answer

Total magnification = 10 × 40 = ×400. The student should use the fine focus wheel.

5. Exam question Describe 4 marks Easier

Describe how to use a light microscope to look at a slide of onion cells.

Mark scheme — 4 marks available

  • Slide on the stage, held by clips — 1 mark
  • Start with the lowest power objective lens — 1 mark
  • Coarse focus to find the image — 1 mark
  • Fine focus to sharpen it — 1 mark

Model answer

Place the slide on the stage and secure it with the clips. Select the lowest power objective lens. Use the coarse focus wheel to bring the image roughly into focus, looking from the side first so the lens does not hit the slide. Then use the fine focus wheel to make the image sharp. Change to a higher power lens and refocus with the fine wheel if more detail is needed.

6. Exam question Explain 4 marks Easier

Explain how electron microscopy has increased our understanding of sub-cellular structures.

Mark scheme — 4 marks available

  • Electron microscopes have higher magnification — 1 mark
  • Electron microscopes have higher resolution / resolving power — 1 mark
  • Shows structures not visible with a light microscope, for example ribosomes or the inside of mitochondria — 1 mark
  • Understanding of how structures work (their functions) has improved — 1 mark

Model answer

An electron microscope has a much higher magnification and resolving power than a light microscope. This means it can show structures that are too small to be seen with a light microscope, such as ribosomes, plasmids and the internal structure of mitochondria and chloroplasts. Scientists have used this detail to work out how the structures carry out their functions.

7. Exam question Calculate 2 marks Easier

An electron micrograph shows a chloroplast that is 25 mm long. The real chloroplast is 5 µm long. Calculate the magnification. Give your answer in standard form.

Mark scheme — 2 marks available

  • Converts 25 mm to 25 000 µm and divides by 5 — 1 mark
  • \(5 \times 10^{3}\) — 1 mark

Model answer

25 mm = 25 000 µm. Magnification = 25 000 ÷ 5 = 5000, which is \(5 \times 10^{3}\).

8. Exam question Calculate 3 marks Easier

A ribosome is 25 nm across. In an electron micrograph the image of the ribosome is 5 mm across. Calculate the magnification. Give your answer in standard form.

Mark scheme — 3 marks available

  • Converts 5 mm to 5 000 000 nm — 1 mark
  • 5 000 000 ÷ 25 = 200 000 — 1 mark
  • \(2 \times 10^{5}\) — 1 mark

Model answer

5 mm = 5 000 000 nm. Magnification = 5 000 000 ÷ 25 = 200 000, which is \(2 \times 10^{5}\).

9. Multiple choice 1 mark Core

What does magnification tell you?

  1. A How clearly two points can be told apart
  2. B How many times bigger the image is than the real object Correct
  3. C How bright the image is
  4. D How fast the specimen is moving

Why: Magnification is how many times bigger the image is than the real object.

10. Multiple choice 1 mark Core

Which microscope has the higher resolution?

  1. A A light microscope
  2. B They are the same
  3. C An electron microscope Correct
  4. D A magnifying glass

Why: An electron microscope has much higher resolving power than a light microscope, so it shows more detail.

11. Multiple choice 1 mark Core

A cell is 2 mm wide in a drawing. The real cell is 0.02 mm wide. What is the magnification?

  1. A ×0.01
  2. B ×10
  3. C ×40
  4. D ×100 Correct

Why: Magnification = image size ÷ real size = 2 ÷ 0.02 = 100, so ×100.

12. Multiple choice 1 mark Core

Which of these can a light microscope show?

  1. A A chloroplast Correct
  2. B A ribosome
  3. C A plasmid
  4. D The folds inside a mitochondrion

Why: A light microscope shows nuclei, cell walls and chloroplasts. Ribosomes and plasmids are too small and need an electron microscope.

13. Multiple choice 1 mark Core

Which lens should you use first when focusing a specimen?

  1. A The highest power objective lens
  2. B The lowest power objective lens Correct
  3. C The eyepiece lens only
  4. D Any lens, because they are the same

Why: Start with the lowest power lens so that you can find the specimen in a wide field of view.

14. Multiple choice 1 mark Core

How many micrometres are there in 1 mm?

  1. A 10
  2. B 100
  3. C 1000 Correct
  4. D 1 000 000

Why: Milli and micro differ by a factor of 1000, so 1 mm is 1000 µm.

15. Multiple choice 1 mark Stretch

A drawing shows a cell as 45 mm wide. The cell is really 15 µm wide. What is the magnification?

  1. A ×3
  2. B ×30
  3. C ×300
  4. D ×3000 Correct

Why: Convert 45 mm to 45 000 µm, then divide by 15 µm to get 3000.

16. Multiple choice 1 mark Stretch

An image is 6 mm wide and the magnification is ×200. What is the real width?

  1. A 3 µm
  2. B 30 µm Correct
  3. C 300 µm
  4. D 1200 µm

Why: Real size = image size ÷ magnification = 6 ÷ 200 = 0.03 mm, which is 30 µm.