Thread by @drskyskull

Let's start a new series of #OpticsLessonOfTheDay! First up: the three main branches of optical physics! I break things down into geometrical optics, physical optics, and quantum optics.

Geometrical optics is the oldest branch, and consists of treating light as propagating in straight lines except for reflection and refraction at interfaces.

Geometrical optics doesn't really consider any "physics" of light, and both the wave theory and the particle theory of light were argued to explain the geometrical properties of light. Huygens argued wave, Newton argued particle; Newton basically "won" in early 1700s.

Geometrical optics is still used today, as "ray tracing" software is used to design complicated optical systems. It has the advantage of being relatively computationally efficient.

"Physical optics" today refers to the wave properties of light, and are usually grouped into interference, diffraction, and polarization. Thomas Young really kicked off the wave era with his double slit experiment, showing wave interference.

Young's experiment, done in early 1800s, showed interference: that waves can enhance or cancel each other out, forming bright and dark bands. He showed this with light passing through two small holes, creating two sources of light.

"Diffraction," first noted by Francesco Maria Grimaldi in the 1600s, is the bending of light around corners or spreading out after passing through small holes. Basically, waves like to spread out as they propagate.

"Polarization" refers to the transverse wave nature of light. The electric and magnetic fields that make up a light wave "wiggle" perpendicular to the direction that light travels.

This transverse wave nature explains why optical calcite shows two images: natural light comes "unpolarized," with both horizontal and vertical wiggles. Each of them refracts a little differently in calcite.

Much of the practical work in optics these days is related to physical optics, as we design optical systems that are small enough so that wave effects are significant and/or that we can take advantage of them.

"Quantum optics" refers to the study of light having a dual nature: sometimes acting like a wave, sometimes acting like a particle. This interpretation became prominent after Einstein explained the photoelectric effect in 1905 using a particle theory of light.

A lot of early work in quantum optics focused on using light to understand quantum physics in general: what, exactly, does it all mean? Researchers use single light particles (photons) to test the predictions of quantum physics and challenge our conventional interpretation.

In recent years, quantum optics has become more important as people push for applications like quantum computing and quantum cryptography. My understanding is that much progress has been made but the promise of quantum effects has not truly been realized yet.

People only use quantum physics when they *really* have to: modeling an ordinary optical system as a system of quantum particles would be cumbersome and is generally unnecessary.

So optical physics is really a mixture of three different paradigms of light, each of which has its uses in particular circumstances! /END

(Images taken from Wikipedia and from my own personal collection.)