A student prepares a temporary mount of onion epidermal cells and observes them using a light microscope. Her eyepiece graticule has 100 divisions. She places a stage micrometer (1 mm divided into 100 divisions) on the stage. At a particular objective magnification, she observes that 50 divisions of the eyepiece graticule exactly cover 40 divisions of the stage micrometer. She then measures the length of an epidermal cell as 25 graticule divisions. She draws this cell, and her drawing measures 75 mm in length.
Describe how the student calibrates the eyepiece graticule, calculates the actual length of the cell in micrometres (µm), and determines the magnification of her drawing. Also describe the differences between light and electron microscopes in terms of resolution and magnification, and explain why electron microscopes are required to study the detailed internal structure of organelles such as mitochondria.