Color is strange, mainly due to perception. Setting aside
complex brain processes, what we see is the result of light absorption,
emission, and reflection. Trees appear green because atoms inside the leaves
are emitting and/or reflecting green photons. Semiconductor LED brake lights
emit single color light when electrical current passes through the devices.
Here’s a question: Can scientists generate any color of
light? The answer is not really, but the invention of the laser in 1960 opened
new doors for this endeavor. An early experiment injected high-power laser
light through quartz and out popped a different color. This sparked the field
of nonlinear optics and with it, a new method of color generation became
possible: frequency conversion.