Cell structure: describe question

A student prepared a temporary mount of onion epidermal cells and observed them under a light microscope. The student also studied a transmission electron micrograph (TEM) of a mitochondrion from a similar cell.

Describe: (a) how the student could use an eyepiece graticule and a stage micrometer to measure the actual length of an onion cell. (b) why the light microscope cannot resolve the internal structure of the mitochondrion, and how the electron microscope achieves greater resolution.