The Physics of Light

xenvy04 (40)in #steemstem • 6 years ago (edited)

The Physics of Light

What is Light?

Light behaves as both a particle and as a wave propagating at a speed of approximately 300,000 km/s. A photon is a "quantum" of this electromagnetic radiation - that is to say, it is the smallest unit through which we can describe this radiation. A photon is an elementary particle with no mass and has both wave-like and particle-like properties (like all of the elementary particles).

How long have we known the speed of light? In fact, as early as 1670, the Danish mathematician Olaus Rømer observed a difference of 11 minutes in the schedule of the eclipses of Jupiter's moons, depending on the distance from Jupiter to Earth. He was able to then calculate the speed of light to be 210,000 km/s, which has an error of 30% from the modern value.

Light will take longer to travel along path B than along path A, creating the delay of ~11 minutes that Rømer observed.

The timing of the Io eclipse was delayed from the expected time (according to Newtonian mechanics) when Earth was further from Jupiter

Newton believed that light was made up of particles which he called "corpuscles." Scientific data showed that this cannot simply be the case: there was ample experimental evidence that light behaved as a wave!

To many, this would sound absurd. We know that sound is a wave, and we know that sound can "turn corners." That is to say, if you and and a friend were standing on opposite sides of a wall and one yelled something at the other, the other would be able to hear it. If I shine a light on an object it creates a shadow, I don't see light on the other side. So it would appear light does not do this, correct? In fact, it actually does!

If you look very closely at a very sharp edged screen, you will see some light goes behind the screen. It is a very small effect, and the light dies out almost as it turn the corner, so it is hard to see (and thus not many people know about this!). Moreover, if the corner is not very sharp, the light gets scattered and the effect disappears.

Shadow cast on the corner of a sharp edge. The shadow is not completely dark, because light got around the corner.

This should also mean that the shadow of a dark circular screen should have a bright spot at its center. Sounds ridiculous, doesn't it? Well, it's actually true!

Shadow cast by a small opaque disk. The bright spot in the center comes from the light that made it around the screen.

With this, it was universally accepted by the scientific community as early as the beginning of the 19th century that light in fact was a wave travelling at large but finite speed.

So light is a wave, similar then to sound waves, or water waves. But all these waves are produced by the undulations of some medium: water for water-waves, air (for example) for sound, etc. Thus it was postulated that the medium in which light undulates is called ether. We now consider it to be a wave of electromagnetic radiation.

So does that mean that Newton's particle theory was totally wrong? Nope! It would not be until 1900 that Max Planck, in attempting to explain black body radiation, would suggest that although light was a wave, those waves could lose or gain energy in finite amounts which he called "quanta" (plural of quantum). In 1905, Einstein applied this idea of light quanta to explain the photoelectric effect, lending more credibility to this particle-like description of light. In 1923, Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons ("Compton scattering") could be explained by a particle-theory of light, but not the wave-theory. Finally, in 1926, Gilbert Lewis named these light quantum particles "photons."

The photoelectric effect that Einstein described. When light hits a surface, electrons are released. Electricity comes from the flow of electrons ("current"). This is the foundation for modern solar technology.

The modern theory of quantum mechanics now describes light as both particle-like and wave-like. Really, the modern understanding is that we can mathematically model light by considering it to be like one macroscopic phenomenon (particles) or another macroscopic phenomenon (ocean waves). But really, neither of these truly describe light; these are merely our human way of trying to understand a phenomenon that we do not currently understand, by modeling it like things that we can see and that we do understand.

We find that light behaves more like a wave at lower frequencies, and more like a particle at higher frequencies, but never completely loses the qualities of one or the other.

Although these models may work to describe light, it is likely that our modern understanding of light is not wholly true or satisfactory, and perhaps one day will be replaced by a new Theory of Light.