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Using a microscope and magnificationIB MYP Sciences: Revision notes

Section 1

Parts of a light microscope

A light microscope uses lenses and light to magnify small specimens.

  • The eyepiece lens is the lens you look through. It is usually ×10.
  • The objective lenses are on a rotating turret. They have different powers, such as ×4, ×10 and ×40.
  • The stage holds the slide.
  • The light source (lamp or mirror) shines light up through the slide.
  • The coarse focus knob moves the stage or lens a large distance. The fine focus knob makes the image sharp.
Key termslight microscopeeyepiece lensobjective lensstage

Section 2

Using the microscope

  1. Turn the objective lens turret to the lowest power.
  2. Place the slide on the stage.
  3. Looking from the side, use the coarse focus to bring the lens close to the slide.
  4. Looking through the eyepiece, use the coarse focus to find the specimen, then the fine focus to make it sharp.
  5. Switch to a higher power and sharpen with the fine focus only.

The lowest power gives the widest field of view, so the specimen is easier to find, and the lens is furthest from the slide, which protects it from cracking.

Total magnification = eyepiece magnification × objective magnification. For example, ×10 × ×40 = ×400.

Key termsfield of view
Common mistake

Total magnification is found by multiplying the two lenses, not adding them. ×10 and ×40 gives ×400, not ×50.

Section 3

Preparing a slide

  1. Place a drop of water on a clean slide.
  2. Use forceps to place a very thin piece of tissue flat in the drop. It must be thin so light can pass through.
  3. Add a drop of stain, such as iodine, to make cell structures stand out.
  4. Lower a cover slip slowly at an angle so that air bubbles are not trapped. The cover slip also keeps the specimen flat.

Safety: handle glass slides carefully, and use eye protection and gloves with stains.

Key termsstaincover slip

Section 4

Drawing scientific diagrams

A scientific drawing records what you see. Follow these rules:

  • Use a sharp pencil and draw clear, single lines. Do not shade or colour.
  • Draw the cells large, using most of the space.
  • Label with straight lines that do not cross, ruled from the structure to the label.
  • Give the drawing a title and show the scale or the magnification.
Key termsscale

Section 5

Magnification and units

Magnification tells you how many times bigger the image is than the real object.

magnification = image size ÷ actual size

Rearranged: actual size = image size ÷ magnification.

Image size and actual size must be in the same unit. Units used:

  • 1 mm = 1000 micrometres (µm)
  • To convert mm to µm, multiply by 1000. To convert µm to mm, divide by 1000.
Key termsmagnificationmicrometre (µm)
Exam tip

Draw a quick triangle with image on top and actual size and magnification below. Cover the one you want to find.

Section 6

Worked example

A cell in a drawing is 40 mm wide. The magnification is ×800.

actual size = 40 ÷ 800 = 0.05 mm

In micrometres: 0.05 × 1000 = 50 µm.

Check the other way: 50 µm = 0.05 mm, and 40 ÷ 0.05 = 800.

Must know

  • Light microscope parts: eyepiece, objective lenses, stage, light, coarse and fine focus
  • Start on the lowest power; coarse focus then fine focus
  • Thin tissue, stain, cover slip lowered at an angle
  • Drawings: sharp pencil lines, no shading, label lines, scale
  • magnification = image size ÷ actual size
  • 1 mm = 1000 µm

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Using a microscope and magnification

  1. A student in Mumbai is using a light microscope for the first time to look at a prepared slide of onion cells. The microscope has an eyepiece lens of magnification ×10, three objective lenses (×4, ×10 and ×40), a stage, a coarse focus knob and a fine focus knob.
    Explain why the student should start by looking at the slide with the lowest-power objective lens.2 marks
  2. A technician in Cape Town prepares a temporary slide of the thin, transparent skin from the inside of an onion bulb. She has forceps, a dropper of water, iodine solution, a microscope slide and a cover slip.
    Describe how the cover slip should be placed on the specimen, and give the reason for this.2 marks
  3. A student in Seoul observes cheek cells under a light microscope and makes labelled drawings. Her teacher asks her to calculate the magnification of each drawing so that other students can work out the real size of the cells.
    One cheek cell in her drawing is 60 mm wide. Its actual width is 0.03 mm. Calculate the magnification of the drawing.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).