Microscopes and magnification

Cells are far too small to see with the naked eye, so we use microscopes. A light microscope uses lenses to make an image bigger, up to about ×1,500. An electron microscope uses a beam of electrons and can magnify far more and show much finer detail. One formula links the size of the image, the real size of the object and the magnification.

Key facts

  • Parts of a light microscope: eyepiece lens, objective lenses, stage, light source, and the coarse and fine focusing knobs.
  • Total magnification = eyepiece magnification × objective magnification: a ×10 eyepiece and a ×40 objective give ×400.
  • Magnification = image size ÷ real size. Rearranged: real size = image size ÷ magnification, and image size = real size × magnification.
  • Use the same units for the image and the real size. 1 mm = 1,000 µm (micrometres) and 1 µm = 1,000 nm (nanometres).
  • Resolution is the smallest distance between two points that can still be seen as separate. Electron microscopes have a much higher resolution, so they show ribosomes and the inside of mitochondria.
  • Light microscopes are cheap and can show living cells in colour; electron microscopes need the specimen to be dead and in a vacuum.
  • To make a slide: take a thin layer of cells, add a stain (iodine for onion cells, methylene blue for cheek cells), and lower a coverslip at an angle so no air bubbles are trapped. Start with the lowest-power objective.

Worked examples

Example 1

A microscope has a ×10 eyepiece and objectives of ×4, ×10 and ×40. What is the highest total magnification?

  1. Total magnification = eyepiece × objective.
  2. The highest power objective is ×40. 10 × 40 = 400
  3. So the highest total magnification is ×400.

Answer: ×400

Example 2

Under a microscope at ×400, the image of a cheek cell is 24 mm across. How wide is the real cell, in µm?

  1. Rearrange magnification = image size ÷ real size to get real size = image size ÷ magnification.
  2. Put in the numbers. 24 ÷ 400 = 0.06 mm
  3. Change mm to µm: multiply by 1,000. 0.06 × 1000 = 60 µm

Answer: 60 µm

Example 3

A drawing of a plant cell is 75 mm long. The real cell is 50 µm long. What is the magnification of the drawing?

  1. The two sizes must be in the same units. Change the drawing to µm. 75 mm = 75000 µm
  2. Magnification = image size ÷ real size. 75000 ÷ 50 = 1500
  3. So the drawing is ×1,500.

Answer: ×1,500

Example 4

On an electron micrograph at ×50,000, a mitochondrion is 100 mm long. Find its real length in mm, in standard form.

  1. Real size = image size ÷ magnification. 100 ÷ 50000 = 0.002 mm
  2. Move the decimal point 3 places to write it in standard form. 0.002 = 2 × 10^(−3)
  3. So it is 2 × 10⁻³ mm, which is 2 µm.

Answer: 2 × 10⁻³ mm (2 µm)