Adding two coherent waves

You will write three small functions that add two coherent waves.

The idea. Two waves of intensities $I_1$ and $I_2$ with a phase difference $\varphi$ give $I = I_1 + I_2 + 2\sqrt{I_1I_2}\cos\varphi$. The greatest value has $\cos\varphi = 1$ and the least has $\cos\varphi = -1$. The visibility of the fringes is $(I_{\max} - I_{\min})/(I_{\max} + I_{\min})$.

Your tasks.

1. In intensity, return the intensity for the phase difference phase_deg in degrees.
2. In extremes, return the pair (greatest, least) of intensities.
3. In visibility, return the visibility of the fringes.