Exercises: Microscope observation

In the light microscope diagram, there are three parts labelled with "?". Write the name of each one and explain its function.

Put the steps for preparing an onion cell sample in order: (a) cover with the coverslip taking care to avoid air bubbles, (b) place a thin layer of onion on the microscope slide, (c) add a drop of iodine solution (lugol) to stain, (d) remove excess liquid with filter paper.

Why are many samples stained before observing them under a microscope? Give an example of a stain used in the school laboratory.

A microscope has an eyepiece lens of $10\times$ and objective lenses of $4\times$, $10\times$, and $40\times$. Calculate the total magnification achieved with each objective lens, and explain which one you should always start observing with and why.

When observing a sample, the image looks very dark and only a fragment of the specimen can be seen. State two possible causes for each problem and what you would do to correct them without changing the objective lens.

Why must you focus first using the lowest power objective lens and reserve the fine focus knob for higher power objective lenses? Explain what might happen if you use the coarse focus knob with the $40\times$ objective lens.

With the $10\times$ objective lens (total magnification $100\times$), the field of view is about $1.5\ \text{mm}$ in diameter. If an onion cell is about $0.25\ \text{mm}$ long, estimate how many cells in a row would fit across the field of view, and explain what will happen to the diameter of the field of view when switching to the $40\times$ objective lens.

A research team wants to film in real time how a unicellular protozoan swims and captures bacteria in a drop of stagnant water, and at the same time, photograph in full detail the internal structure of the viruses that infect those bacteria. A student proposes using only a transmission electron microscope to achieve both goals. Evaluate their proposal with reasoned arguments: explain why it is not possible to observe the live protozoan with this device and determine which microscope should be used in each case and why.

During a practical activity observing cheek cell specimens stained with methylene blue, a group can only see shapeless blue smudges. Suggest at least three errors they might have made when preparing the sample and explain how you would repeat the experiment to get good results.