A grating calculator

You will write three functions for a diffraction grating.

The idea. A grating with $N_1$ lines in each millimetre has the line distance $d = 10^{6}/N_1$ nm. The bright orders are at $\sin\theta = m\lambda/d$. An order exists only if this is less than $1$.

Your tasks.

1. In spacing, return the distance between the lines in nm.
2. In order_angle, return the angle of order m in degrees, or None if the order does not exist.
3. In highest_order, return the highest order that can be seen. An order with $\sin\theta$ exactly $1$ is not seen.