Theories of the Universe: The Dual Nature of Light

The Dual Nature of Light

Light enables us to see and comprehend the universe and is an important part of any study of cosmology. But what kind of thing is it? While it is first and foremost a sensation in the eye, it also has an independent existence outside of us. Newton thought that it was a beam of particles, but what kind of particles? What are they made of? What size are they? What shape? These questions went unanswered until Einstein and a few other physicists arrived on the scene.

By the end of the nineteenth century everyone had conceded that Newton was wrong and that light was a wave, but what kind of wave? An ocean wave is not a thing, it is a property of water, something that water does. If there is no water there is no wave. So if light was a wave, what was waving? This was the most urgent question that physicists were asking. By the time an adequate answer was found, light would end up being described as both a particle and a wave. Yet how could it be both? This would be the first of many paradoxes that would begin to question our common sense notion of how the universe operates.

Waves of Light

While the nineteenth century was filled with many noteworthy scientific discoveries, one of the most significant was the description of light and its properties. The insights that the study of light provided were later crucial to quite a few of Einstein's theories and also formed the basis upon which the big bang theory developed. The key realization that led to all of this was that light was an electromagnetic wave. Let's take a look at how this discovery was made.

About 100 years after Newton's theory about the particle structure of light, a man by the name of Thomas Young performed a very famous experiment in which he showed that light propagated as a wave.

In Young's experiment, a light source was shown on a screen that had two holes a few millimeters apart. He put another screen behind the first, and the light coming through the two holes of the first screen illuminated this second target screen. As expected, two patches of light appeared. He then made the holes smaller and the corresponding patch of light became smaller, too.

But then something very unusual happened. When Young made the holes very small, faint rings appeared around the patches on the target screen that actually made them bigger. Instead of the patches reducing in size to correspond with the smaller holes, they were larger. This couldn't happen if light were made of particles, because particles move in straight lines and wouldn't make these larger faint rings of light around the patches.

If he made the holes even smaller, the patches of light on the target screen began to overlap and became crossed with dark lines. These dark lines were caused by waves of light interfering with one another.

You can get a good idea of just how this works if you drop a couple of rocks in water. The ripples sent out by each rock hitting the water interact with each other. Some cancel each other out and some amplify each other. The light and dark bands seen on the screen are the result of light waves doing the same thing. The dark band is the absence of light, or when light waves cancel each other out, while the lighter bands are where light waves amplify each other. When light waves are forced to travel through very small areas, like the two tiny holes in Young's experiment, the interference pattern that is created can only result if light is a wave, not a particle. Although many scientists scoffed at Young's experiment he was later exonerated by the work of two other physicists, James Maxwell and Heinrich Hertz.