Ok thanks for that.
Interesting on the phase change.
Right, phase changes close to the resonance can be messy to explain.
1. Far from resonant frequency, the energy absorbed is small and the phase change is small.
2. As it approaches the resonant frequency, amplitude increases and the phase difference increases.
3. At some point close to the resonant frequency, the phase difference becomes max (positive or negative) but the amplitude continues to increase.
4. At the resonant frequency the amplitude peaks and the phase change becomes zero.
5. After the peak, the phase difference crosses to the other side (+ve to -ve or vice-versa).
The dispersion spectrum has two peaks on opposite sides and is zero at the point corresponding to the peak in the absorption spectrum.
Optical rotation in molecular spectroscopy is related to the phase difference between two circularly polarized light.
Quadrature detection in electronics is also related.