Light emitting diodes (LEDs) ranging from high-power, high efficiency ones for solid-state lighting to tiny micro-LEDs used for stimulation of nerve cells in the field of optogenetics as well as laser diodes in the violet-blue-green part of the light spectrum have one thing in common: they are based on spontaneous electroluminescence and stimulated light emission in quantum wells made from the semiconductor Indium-Gallium-Nitride (InGaN). Due to their crystal symmetry, all group-III-nitrides (AlN, InN, GaN) and their compounds are ferroelectric, piezoelectric, and optical birefringent. I will discuss these effects and their impact on the physical properties of the above mentioned devices.