A single-slit calculator

You will write three small functions for a single slit.

The idea. The dark places of a single slit are at $y_n = n\lambda D/a$. The central maximum is twice as wide as the distance to the first dark place. The intensity as a fraction of the centre is $(\sin\alpha/\alpha)^2$, with $\alpha = \pi a\sin\theta/\lambda$, and it is $1$ at $\alpha = 0$.

Your tasks.

1. In central_width, return the width of the central maximum in mm. The wavelength is in nm, the slit width a in mm and the distance D in m.
2. In minimum_position, return the distance of dark place number n from the centre, in mm.
3. In relative_intensity, return $(\sin\alpha/\alpha)^2$, and return $1$ when $\alpha$ is $0$.