A student uses a light microscope to observe a specimen of onion epidermis. She calibrates the eyepiece graticule using a stage micrometer that has a scale of 1 mm divided into 100 divisions. She determines that at the chosen magnification, 50 divisions on the eyepiece graticule correspond to 0.25 mm on the stage micrometer. She then uses the eyepiece graticule to measure the length of a single onion epidermal cell. The cell spans 8 divisions on the eyepiece graticule.
Describe how the student calibrates the eyepiece graticule and uses it to determine the actual length of the cell in micrometers. Include in your description: • the meaning of calibration and why it is necessary • how the stage micrometer is used to find the value of one eyepiece division • the calculation of the actual length of the cell in micrometers • how the length could also be expressed in nanometers • why the calibration must be repeated if the magnification is changed