A thin film and a sheet

You will write three functions for thin films and for a sheet over a slit.

The idea. A film in air is bright in reflection when $2nt = (m + \tfrac12)\lambda$. A coating of index $n$ on glass cancels the reflection of $\lambda$ when $t = \lambda/4n$. A sheet of index $\mu$ and thickness $t$ over one slit shifts the pattern by $(\mu - 1)tD/d$.

Your tasks.

1. In ar_thickness, return the least thickness in nm of a coating of index n that cancels the reflection of wavelength in nm.
2. In visible_bright, return a list of the wavelengths in nm, from $400$ to $700$, that a film of index n and thickness t in nm reflects strongly. Round each to $1$ decimal and give the list from the shortest to the longest.
3. In sheet_shift, return the shift of the pattern in mm. The thickness t is in micrometre, D in m and d in mm.