Fizeau Method

In an experiment to measure the speed of light by Fizeau's apparatus, the following data are used.

Distance between the mirrors$=12.0 \mathrm{km}$,

the number of teeth in the wheel$=180$.

Find the minimum angular speed of the wheel for which the image is not seen in ${10}^4 deg s^{-1}$.

In the original Fizeau method, the light travelled $8.6$ $\mathrm{km}$ and then returned. What could be the difficulty if this distance is taken as $8.6 \mathrm{m}$

In Fizeau method of measuring the speed of light, the toothed wheel is placed in the focal plane of a converging lens. How would the experiment be affected if the wheel is slightly away from the focal plane?