What is light?

Allen Versfeld's picture
 

Dear Urban Astronomer

 
I just read your article about "what is the speed of light."  I'm so surprised (and amazed) that light travels faster in a vacuum than in (atmosphere?) - does this mean that LIGHT is actual..."stuff" ? It might sound silly to you, but I find it quite hard to comprehend! Does this mean that light has a "mass"?  It's very cool, and opens up a whole new world of symbolic drama-movie themes.
 
Jody
 
 
Dear Jody
 
What you're asking here is one of the great mysteries of the universe.  Light is quite obviously a real thing, as anybody with the gift of sight could tell you, but figuring out what kind of thing has turned out to be quite difficult.  Philosphers (and later, scientists) have been grappling with this question for centuries, but it's only in the last hundred years that physicists really got a proper answer.  I'm afraid you're not going to like it when you hear it, since it goes against all common sense.
 
The ancient Greek philosophers had some ideas, which were interesting as much for their variety as for their inventiveness.  Empedocles wrote in the 5th century BC that when Aphrodite designed the human eye, she placed the element of fire inside it which would send light out making it possible for the eye to see.  Despite the obvious logical flaw (So why can't we see in the dark, then?) this idea really stuck in the popular consciousness - it remained the dominant theory for almost two thousand years.  In fact, the first real progress in understanding the nature of light came from the Arab world at around the year 1000 AD, when the Islamic mathematician Abu Ali al-Hasan ibn al-Haytham began to study optics.  He argued convincingly that the eye did not project a beam, and saw purely by external light passing in through the lens.  A central part of his argument was the construction of a Camera Obscura, which clearly demonstrated how an image could be created of distant objects in a purely passive way, by permitting external light to pass through a small hole.  It would take six hundred years for his work to become known in Europe, and it was shortly afterwards that the wave theory of light first appeared

Wave Theory

As more and more people took to studying light, they began noticing some interesting properties.  Light always wants to travel in a straight line, but when it strikes a mirror, it will reflect away.  The angle at which it reflects away will always be exactly equal to the angle at which is struck, and this is exactly what common sense and daily experience tells us will happen.  But when it passes from one medium to another (such as when a beam of light aimed at a pool passes from the air into the water), it changes direction in a way that is a little less obvious.  You can see the effect by placing a stick in a bowl of water:  it seems to bend at the point where it enters the water.  This phenomenon is called Refraction, and is how lenses work and is responsible for a number of optical effects in our atmosphere.
Light has a third property which is far less obvious to casual observers:  Diffraction.  If you shine a light through a narrow slit towards a screen, you'll notice something very strange happen:  Instead of just casting a narrow line of light onto the screen, you get a line with a pair of fainter lines on either side, and another even fainter pair of lines further out.  Diffraction also describes the tendency of light to bend slightly when passing an edge, so that shadows always have blurry edges.  The classical idea of light being some sort of ray, or beam of particles, cannot possible explain this phenomenon, but there is something that everybody is very familiar with that has all three properties of reflection, refraction and diffraction:  waves.  Whether they'rs soundwaves, earthquake shockwaves or even waves in water, these phenomenon are easily demonstrated and well understood.  The only problem with seeing light as a wave is that waves don't exist in a vacuum, but must travel through a medium (be it water, air, the earth, guitar strings, electrical currents, or whatever).  Still, the behaviour of light matched that of light so closely that the generaly accepted assumption was that there must be some sort of invisible intangible medium called Aether which permeated the universe and allowed light waves to travel.
 

Particle Theory

Philosophers going as far back as the ancient Greeks had always seen a beam of light as being composed of miniscule particles of something.  The idea appeals to common sense, but the success of the wave theory had convinced all educated people that it was wrong.  Then, towards the end of the 19th century, as the gentleman scientists began their age of enlightenment, new properties of light began to be discovered.  It was discovered that a beam of light shining on certain metals could generate an electric charge, a phenomenon known as the Photoelectric Effect.  When light strikes the surface of a metal, the loosely associated electrons near the surface get knocked free, causing a charge to accumulate.  Common sense suggested that the energy of the electrons should be affected by the brightness of the light.  After all, according to wave theory, a brighter light would mean a more energetic wave in the aether.  But experiments revealed that while increasing the brightness resulted in more electrons being ejected, their energy did not increase.  Instead, and to everybody's surprise, it was the colour of the light that affected the energy.  This directly contradicted wave theory, and could only be explained by viewing light as a stream of particles, with different colours having particles of different energy.  This was a problem, because there were now two theories that both perfectly explained some of the aspects of light, but were still completely contradicted each other.  The solution was found in 1905 by an employee of the Bern patent office who happened to be a particularly inspired student of physics.  His name was Albert Einstein, and he wrote a paper suggesting a way for both theories to be true.
 

Wave-Particle Duality

Einstein's paper, published in 1905 and establishing his reputation as a world-class physicist, suggested that light might indeed be composed of a stream of particles, but that these particles (called photons) were actually little packets of energy.  These photons behave like waves, with the equivalent frequency being proportional to the amount of energy contained within each photon.  In other words, a stream of photons are distinct particles that nevertheless behave like waves.  Later experiments (including one where individual photons were fired through a double-slit one at a time, and still managed to create a wave-like interference pattern despite there being nothing for the individual photons to interfere with) proved that this bizarre idea was actually true, and Einstein's work proved fundamental to the seemingly nonsensical (yet absolutely real) field of Quantum Mechanics.
 
So in summary, light violates common sense.  It is a particle that is a wave, but isn't a "real" thing in the common sense of the world, and the wave doesn't exist.  Aren't you sorry you asked?
 
 
 
 

Written by Allen Versfeld
Comments?  Questions?  Why not mail me at uastronomer@gmail.com

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