Starter
• You are observing a specimen of squamous tissue under
high power.
• Each individual cell has an average diameter of 60mm
and the diameter of the field of view is 2mm.
• Calculate the maximum number of cells that are visible
in the field of view.
Calculating Cell Size
Measuring objects, using a
graticule
(PAG 1 and M1.8, M2.2)
Objectives and Success Criteria
Objectives
• Use the magnification
formula
Success criteria
• Use and manipulate the
magnification formula
• Examine and draw
specimens
• Prepare a specimen for
observation and measure
organelles and cells.
Key Terms
• Resolution: the ability to distinguish two
separate points as distinct from each other
• Magnification: the number of times
greater an image is than the object
Practicing magnification maths
• There are:
• 1 000 000 nanometres (nm) in a millimetre (mm)
• 1000 micrometres (mm) in a millimetre
• 1000 millimetres in a metre (m)
• 1 000 000 micrometres in a metre
• 1 000 000 000 nanometres in a metre.
Calculating Actual Size
• Actual size = image size
magnification
• Magnification = image size
actual size
Worksheet
Worked Example
Image size is 15mm (15 000 mm)
15 000/2.6 = 5769 times
Eye Piece Graticule
• Microscopes can be fitted with an EPG
• Ruler etched on it
• A specimen can be measured in eyepiece units (an
arbitrary measurement, the image changes size
depending on the magnification, but the graticule stays
the same size for each magnification.
• To measure the size of objects in the field of view, the
graticule needs to be calibrated)
Calibration - Using Stage
Micrometer
• Place a stage graticule on the stage
• The ruler is 1mm long and split into 100
divisions
• Each division = 10µm (0.01mm)
• 1µm is equal to 1 millionth of a metre
Calibration - Using Stage
Micrometer
• Align the eyepiece graticule with the stage
micrometer
• Find the value of one eyepiece division
• (1mm or 1000mm)
In (a) where mag = x40,
The stage graticule is equal to 40
eyepiece divisions.
Each eyepiece division =
1000mm/40 = 25mm
In (b) where mag = x100
The stage graticule is equal to 100
Remember the stage graticule is 1mm or 1000mm) epd
Each epd = 1000mm/100 = 10mm
Calculating size
• In the image (a), the nucleus is
3.2epd
With x100 mag. 1epd = 10mm
So: the nucleus is 3.2x10 = 32mm
Look at your graticule and stage
Micrometer.
Use them to calculate the size of
an onion cell and a cheek cell.
Calculate Cell Size
• Prepare an onion cell slide
• Measure the size of a cell
Plenary Question
1. If a nucleus measures 100mm on a
diagram, with a magnification of x10000,
what is the actual size of the nucleus?
Homework
• Questions 1-5 p34 purple book
• Or Flipped learning – Fill in organelle work
sheet in folder.