Magnification And Calibration

Magnification

  • Magnification is how many times larger the image is than the actual size of the object being viewed. Can be adjusted by interchangeable ‘Objective lenses’

  • Calculation for magnification:


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Magnification = size of Image/Actual size of object

All have to be in the same unit of measurement

size of image = length of image measures using a ruler

Different Measurements:

  • 1000 nanometres (nm)=1 micrometre (um)

  • 1000 micrometres (um) = 1 mm

  • 1000mm = 1 metre

Using a graticule to calibrate a light microscope

  • to measure size of sample under microscope, must use an eyepiece graticule - very small scale located within eyepiece lens, has arbitrary units (no units on the ruler)

  • Use stage micrometre- a slide with scale marked on it (1mm=100 divisions) (1 division=0,01mm(10um)), Placed on the stage of microscope.

  1. Adjust micrometre with eyepiece graticule to align. Example - 50 micrometre divisions = 87 graticule divisions (0.5mm=87 divisions)

  2. 0.5mm/87=0.00575mm → 1 division on eyepiece graticule = 0.00575mm

  3. Successfully calibrated to measure actual size of sample

Resolution

  • Resolution = ‘ability to see individual objects as separate entities.’

  • Resolution increases the detail that can be seen through the magnification.

  • Resolution is limited by diffraction of light as it passes through the sample (and lenses)

  • Diffraction = tendency of light waves to spread as they pass close or through physical structures eg the specimens being studied.

  • The structures present in the specimens are very close to each other therefore light reflected may be overlapping (due to diffraction) - leads to unclear resolution.

  • Resolution increased by using Electron microscopes which have much higher resolution because electrons have wavelengths x1000’s shorter than visible light, so they produce far less diffraction and can distinguish much smaller structures.