The lens, the slab and the prism in code

You will write four functions for the optics experiments and the meters. They follow the lesson.

The idea. For a real image, $f = \dfrac{uv}{u + v}$. For a slab, $n = \dfrac{\sin i}{\sin r}$. For a prism at the least deviation, $n = \dfrac{\sin\frac{A + \delta_m}{2}}{\sin\frac{A}{2}}$. A voltmeter made from a galvanometer needs a series resistor $R = \dfrac{V}{I_g} - G$.

Your tasks.

1. In lens_focal, return $f$ from the object distance u and the image distance v.
2. In slab_index, return $n$ from the angles i and r in degrees.
3. In prism_index, return $n$ from the prism angle A and the least deviation dm, both in degrees.
4. In voltmeter_resistor, return the series resistor for the range V in volt, the galvanometer current Ig in A and the resistance G in ohm.