AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Measuring the size of an object viewed with an optical microscope.

    Full specification wording

    Methods of studying cells: Measuring the size of an object viewed with an optical microscope.

    Size is measured with an eyepiece graticule, a scale etched into the eyepiece whose divisions have no fixed value until they are calibrated.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 2 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Size is measured with an eyepiece graticule, a scale etched into the eyepiece whose divisions have no fixed value until they are calibrated. You calibrate by viewing a stage micrometer, a slide carrying an accurate scale, usually one millimetre divided into one hundred divisions of ten micrometres. Line the two scales up, count how many eyepiece divisions cover a known length on the micrometer, and divide to find the real value of one eyepiece division at that objective. The calibration holds only for that objective, so it must be repeated whenever you change lens. Then count the eyepiece divisions spanning the specimen and multiply.

    What earns the marks

    • Measuring the image accurately and converting it to the same unit as the real object, for example 46 mm recorded as 46 000 micrometres.
    • Correct use of magnification equals size of image divided by size of real object, shown as the division even if the arithmetic is wrong.
    • A final magnification consistent with the measurement, quoted without units, for example about 20 000 times.
    • Calibrating an eyepiece graticule against a stage micrometer, and recalibrating after changing the objective lens.
    • Rearranging the formula correctly when the real size rather than the magnification is the unknown.

    Common mistakes and how to put them right

    • Dividing the real size by the image size, which inverts the magnification.

    • Leaving the image in millimetres and the object in micrometres, so the answer is out by a factor of one thousand.

    • Attaching a unit to a magnification, which is a ratio and has none.

    • Treating eyepiece graticule divisions as fixed micrometres without calibrating against a stage micrometer.

    • Measuring the image carelessly.

      Put it right: The mark scheme allows only a narrow band of ruler measurements, so a millimetre either way can cost the answer mark.

    Exam tips

    • Always show the conversion and the division; AQA awards a mark for correct use of the equation or the correct image size in micrometres even when the final answer is wrong.
    • Convert everything to micrometres before dividing, then check the conversion rather than the size of the answer: a printed image has been enlarged far beyond what the microscope itself did, so an optical-microscope image of a bacterium can legitimately work out at about 20 000 times.
    • Measure to the ends the question specifies, for instance excluding a flagellum if the stem says so.

    Try the lesson

    The start of the Methods of studying cells: Measuring the size of an object viewed with an optical microscope. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A specimen is viewed at a total magnification of ×400. The image of a cell measures 20 mm. What is the actual length of the cell?

    Listen

    Size is measured with an eyepiece graticule, a scale etched into the eyepiece whose divisions have no fixed value until they are calibrated. You calibrate by viewing a stage micrometer, a slide carrying an accurate scale, usually one millimetre divided into one hundred divisions of ten micrometres. Line the two scales up, count how many eyepiece divisions cover a known length on the micrometer, and divide to find the real value of one eyepiece division at that objective. The calibration holds only for that objective, so it must be repeated whenever you change lens. Then count the eyepiece divisions spanning the specimen and multiply. A strong answer earns marks for these points. measuring the image accurately and converting it to the same unit as the real object, for example 46 mm recorded as 46 000 micrometres. correct use of magnification equals size of image divided by size of real object, shown as the division even if the arithmetic is wrong. a final magnification consistent with the measurement, quoted without units, for example about 20 000 times. calibrating an eyepiece graticule against a stage micrometer, and recalibrating after changing the objective lens. Exam tip: Always show the conversion and the division; AQA awards a mark for correct use of the equation or the correct image size in micrometres even when the final answer is wrong.

    See it

    A top-to-bottom flowchart. The first box says view stage micrometer, which leads to line up the two scales. That leads to count eyepiece divisions covering a known length on the micrometer, then to divide to find the real value of one eyepiece division at that objective. From there one branch goes to calibration holds only for that objective, which leads to repeat whenever you change lens. A second branch from the divide step goes to count eyepiece divisions spanning the specimen, which leads to multiply to find the size.
    The sequence for calibrating an eyepiece graticule against a stage micrometer and then measuring a specimen.

    Key words

    Say what each word means before you turn the card.

    eyepiece graticule

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    stage micrometer

    Try answering aloud or in your head before turning the card.

    magnification

    Try answering aloud or in your head before turning the card.

    calibration

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    See every definition
    eyepiece graticule
    A scale etched into the eyepiece whose divisions have no fixed value until they are calibrated.
    stage micrometer
    A slide carrying an accurate scale, usually one millimetre divided into one hundred divisions of ten micrometres.
    magnification
    Size of image divided by size of real object, a ratio that has no units.
    calibration
    Lining the two scales up, counting how many eyepiece divisions cover a known length on the micrometer, and dividing to find the real value of one eyepiece division at that objective.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart. The first box says view stage micrometer, which leads to line up the two scales. That leads to count eyepiece divisions covering a known length on the micrometer, then to divide to find the real value of one eyepiece division at that objective. From there one branch goes to calibration holds only for that objective, which leads to repeat whenever you change lens. A second branch from the divide step goes to count eyepiece divisions spanning the specimen, which leads to multiply to find the size.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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