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AS Biology - How to calibrate a microscope
Jo Phillips A Level Biology
Overview
This video explains the process of calibrating an eyepiece graticule on a microscope, a crucial step for accurately measuring microscopic objects. It details why calibration is necessary due to changes in apparent size with magnification and demonstrates how to use a stage micrometer to determine the real-world value (in millimeters or micrometers) of each division on the eyepiece graticule. The process is shown for two different magnifications, highlighting that calibration must be repeated for each objective lens to ensure accurate measurements of specimens like cells.
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Chapters
- An eyepiece graticule is a scale built into the microscope's eyepiece for measuring objects.
- The apparent size of an object changes with magnification, but the eyepiece graticule's scale does not.
- Therefore, one unit on the eyepiece graticule represents a different real-world distance at different magnifications.
- Calibration is essential to determine the actual size of an object by converting eyepiece graticule units to millimeters or micrometers for each specific magnification.
Without calibration, measurements taken using the eyepiece graticule would be inaccurate and misleading, as the value of each unit changes with magnification.
A cell appears to be one unit long on the eyepiece graticule at low magnification, but the same cell appears to be a different number of units long at high magnification, even though the cell's actual size hasn't changed.
- A stage micrometer is a specialized slide with a known, precise scale, used as a reference.
- It has divisions that are accurately measured (e.g., 0.1 mm between major lines, further divided into 0.01 mm or 10 micrometers).
- Unlike the eyepiece graticule, the stage micrometer's scale is magnified along with the specimen when magnification is increased.
- This allows for direct comparison between the eyepiece graticule and the known scale of the stage micrometer.
The stage micrometer provides the essential known measurement standard required to translate the arbitrary units of the eyepiece graticule into actual, quantifiable lengths.
A glass slide with a tiny scale marked on it, where the distance between major lines is precisely 0.1 millimeters, is placed on the microscope stage.
- Place the stage micrometer on the microscope stage and align the eyepiece graticule and stage micrometer scales.
- Start by aligning the leftmost divisions of both scales.
- Find another point further along where divisions on both scales line up perfectly.
- Measure the distance on the stage micrometer that corresponds to a specific number of eyepiece graticule units.
- Calculate the value of one eyepiece graticule unit by dividing the known stage micrometer distance by the number of eyepiece units.
This systematic alignment and measurement process ensures an accurate conversion factor between the eyepiece graticule units and real-world measurements for a specific magnification.
Aligning the '0' mark of the eyepiece graticule with a mark on the stage micrometer, then finding where the '10' mark on the eyepiece graticule aligns with a specific division on the stage micrometer, indicating that 10 eyepiece units equal 0.99 mm.
- Once calibrated for a specific magnification, remove the stage micrometer.
- Replace it with the specimen slide (e.g., a cell).
- Use the eyepiece graticule to measure the specimen's length in eyepiece units.
- Multiply the measured eyepiece units by the previously calculated calibration factor (value per unit) to find the specimen's actual size.
- Calibration must be repeated if the objective lens (and thus magnification) is changed.
This allows for accurate, quantitative measurement of biological structures or other microscopic objects using the calibrated eyepiece graticule.
Measuring a cell that spans exactly one eyepiece graticule unit, and knowing from the previous calibration (at 40x total magnification) that one unit is 99.9 micrometers, thus determining the cell is 99.9 micrometers long.
- Repeat the calibration process when changing to a higher magnification (e.g., from 40x to 100x total magnification).
- The stage micrometer will appear magnified, and its divisions will span more eyepiece units.
- Align scales, find matching divisions, and calculate the new value of one eyepiece graticule unit.
- The value per eyepiece unit will be smaller at higher magnifications.
- Measurements of the same specimen at different magnifications should yield similar actual sizes, confirming calibration accuracy.
Ensures that measurements remain accurate across different objective lenses, acknowledging that the relationship between eyepiece units and real-world distance changes with magnification.
At 100x magnification, 2.6 eyepiece units might correspond to one 0.1 mm division on the stage micrometer, leading to a new calibration factor of approximately 38.46 micrometers per eyepiece unit.
Key takeaways
- Microscope calibration is essential because the apparent size of objects changes with magnification, while the eyepiece graticule scale remains constant.
- A stage micrometer, with its precisely known scale, is used to calibrate the eyepiece graticule.
- Calibration must be performed for each objective lens (magnification) used, as the value of each eyepiece graticule unit differs.
- The process involves aligning the eyepiece graticule with the stage micrometer and calculating how many micrometers or millimeters each eyepiece unit represents.
- Once calibrated, the stage micrometer is removed, and the eyepiece graticule can be used to accurately measure the size of specimens.
- Accurate measurements rely on repeating the calibration process whenever the magnification is changed.
Key terms
Eyepiece graticuleStage micrometerCalibrationMagnificationObjective lensField of viewMicrometerMillimeter
Test your understanding
- Why is it necessary to calibrate an eyepiece graticule for each different objective lens on a microscope?
- What is the role of the stage micrometer in the calibration process?
- Describe the steps involved in aligning the eyepiece graticule and stage micrometer to perform a calibration.
- How does the calculated value of one eyepiece graticule unit change when you switch from a low-power objective lens to a high-power objective lens?
- Once a microscope is calibrated at a specific magnification, how can you use the eyepiece graticule to determine the actual size of a cell?