B1.1 The World of the Microscope

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: light and electron microscopes, magnification, resolution, scale bars and units of size. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the main parts of a light microscope and work out its total magnification
• Explain the difference between magnification and resolution
• Compare light and electron microscopes, and explain how electron microscopes changed our understanding of cells
• Use magnification = size of image ÷ size of real object, rearrange it and use a scale bar
• Convert between mm, µm and nm and write very small sizes in standard form

Key words

• Magnification: How many times bigger the image is than the real object.
• Resolution: The smallest distance between two points that can still be seen as two separate points. Better resolution means more detail.
• Sub-cellular structure: A part inside a cell, such as the nucleus, a mitochondrion or a ribosome (also called an organelle).
• Micrometre (µm): One thousandth of a millimetre: 1 µm = 0.001 mm = 1 × 10⁻⁶ m.
• Nanometre (nm): One thousandth of a micrometre: 1 nm = 0.001 µm = 1 × 10⁻⁹ m.

The light microscope

Light passes up through a thin specimen on a slide, then through two lenses: the objective lens (near the specimen) and the eyepiece lens (near your eye). Each lens magnifies the image.

• Eyepiece lens; What it does Magnifies the image again (often ×10).
• Objective lenses; What it does Usually ×4, ×10 and ×40. Always start with the lowest power.
• Stage; What it does Holds the slide. Clips stop it moving.
• Lamp; What it does Shines light up through the specimen.
• Coarse and fine focus; What it does Move the stage (or lens) to bring the image into focus. Use the fine focus at high power.

The total magnification is found by multiplying the two lens powers together.

Worked example: total magnification

A student uses a ×15 eyepiece with the ×40 objective lens.
Total magnification = eyepiece × objective = 15 × 40

Answer: ×600

Magnification and resolution

Making an image bigger is only useful if you can also see more detail. Resolution (resolving power) is the ability to tell two points close together apart. A light microscope can only resolve points about 200 nm apart. Past that, extra magnification just gives a bigger blur.

An electron microscope uses a beam of electrons instead of light. Electrons have a much smaller wavelength than light, so the resolution is about 1000 times better.

• Maximum magnification; Light microscope about ×1500 to ×2000; Electron microscope about ×2 000 000
• Resolution; Light microscope about 200 nm; Electron microscope about 0.2 nm
• Specimens; Light microscope living or dead; can be stained and seen in colour; Electron microscope dead only (inside a vacuum); black and white images, colour added later
• Cost and size; Light microscope cheap, small, portable; Electron microscope very expensive, large, needs a special room and training
• What you can see; Light microscope cells, nucleus, cell wall, chloroplasts, vacuole; Electron microscope all of these plus mitochondria, ribosomes and membrane detail

Key fact: Electron microscopes have much higher magnification and resolution. This let scientists see sub-cellular structures such as mitochondria and ribosomes in detail, and study the inner membranes of chloroplasts and mitochondria for the first time.

Calculating magnification

Use this equation. You need to recall it for the exam.

magnification = size of image ÷ size of real object

Both sizes must be in the same unit before you divide. Magnification has no unit, but it is often written with a × sign.

Worked example: finding the magnification

A drawing of a yeast cell is 24 mm wide. The real yeast cell is 8 µm wide.
Same units: 8 µm = 8 ÷ 1000 = 0.008 mm
Magnification = 24 ÷ 0.008

Answer: ×3000

Worked example: finding the real size

In an electron micrograph a chloroplast is 30 mm long. The magnification is ×6000.
Real size = size of image ÷ magnification = 30 ÷ 6000 = 0.005 mm
Convert to µm: 0.005 × 1000 = 5 µm

Answer: 5 µm

Many micrographs have a scale bar instead of a magnification. Measure the bar in mm, convert the length it represents to mm, then divide.

Worked example: using a scale bar

A scale bar on a photo is 18 mm long and is labelled 6 µm. A cell on the photo is 54 mm long.
Magnification = 18 mm ÷ 0.006 mm = ×3000
Real length of cell = 54 ÷ 3000 = 0.018 mm = 18 µm

Answer: ×3000 and 18 µm

Units of size and standard form

Cells are tiny, so we use smaller units. Each step down is 1000 times smaller.

• millimetre; Symbol mm; In metres 1 × 10⁻³ m; Conversion 1 m = 1000 mm
• micrometre; Symbol µm; In metres 1 × 10⁻⁶ m; Conversion 1 mm = 1000 µm
• nanometre; Symbol nm; In metres 1 × 10⁻⁹ m; Conversion 1 µm = 1000 nm

Exam tip: Going to a smaller unit: multiply by 1000. Going to a bigger unit: divide by 1000.

Worked example: standard form

A virus is 120 nm across. Write this length in metres, in standard form.
1 nm = 1 × 10⁻⁹ m, so 120 nm = 120 × 10⁻⁹ m
Move the decimal point two places: 1.2 × 10⁻⁷ m

Answer: 1.2 × 10⁻⁷ m

Two sizes differ by one order of magnitude when one is about 10 times the other (10¹). A 100 µm plant cell is two orders of magnitude bigger than a 1 µm bacterium (10² times).

Common mistakes

• Dividing two sizes that are in different units. Always convert first (usually to mm or µm).
• Thinking more magnification always means more detail. Detail depends on resolution.
• Adding the lens powers. Total magnification = eyepiece × objective (15 × 40 = 600, not 55).
• Saying electron microscopes can show living cells. Specimens must be dead because they are in a vacuum.
• Putting the equation the wrong way up. The image is the bigger number, so magnification = image ÷ real.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows a light microscope. Which letter labels the objective lens? (Choose from: A, B, C, E.)

Answer: B

Working: B is the objective lens, just above the slide. A is the eyepiece lens, C is the stage and E is the coarse focus knob.

Question 2. A drawing of a cheek cell is 35 mm wide. The real cell is 0.05 mm wide. Calculate the magnification of the drawing.

Answer: 700 ×

Working: 35 ÷ 0.05 = 700, so the magnification is ×700.

Question 3. A microscope has a ×15 eyepiece lens and a ×10 objective lens. What is the total magnification?

Answer: 150 ×

Working: 15 × 10 = ×150.

Related lessons

• Next lesson: B1.2 Animal and Plant Cells
• In this chapter: B1.3 Eukaryotic and Prokaryotic Cells
• In this chapter: B1.4 Specialisation in Animal Cells
• In this chapter: B1.5 Specialisation in Plant Cells

Continue in the full course

This lesson is section B1.1 (AGCSC0101) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.