Let There Be Light

When God decided to speak for the first time, He uttered this phrase.  I guess He could see in the dark, as God had already created the heaven and the earth and water.  I always wondered who God was talking to when He said this, at any rate after God commanded that there should be light, the light appeared and this separated the darkness.  The exact nature of visible light is a mystery that has puzzled man for centuries.

Greek scientists from the ancient Pythagorean discipline postulated that every visible object emits a steady stream of particles, while Aristotle concluded that light travels in a manner similar to waves in the ocean.  Even though these ideas have undergone numerous modifications and a significant degree of evolution over the past 20 centuries, the essence of the dispute established by the Greek philosophers remains to this day.  One point of view envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock.  The opposing view holds that light is composed of a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle.  During the past few centuries, the consensus of opinion has wavered with one view prevailing for a period of time, only to be overturned by evidence for the other.  Only during the first decades of the twentieth century was enough compelling evidence collected to provide a comprehensive answer, and to everyone’s surprise, both theories turned out to be correct, at least in part.

The ancient Greeks believed that before this world came into existence, only a confused mass of shapeless elements was present, and that was the primordial god called Chaos.  Chaos was the lower atmosphere which surrounds the earth, including both the invisible air and the gloom of fog and mist.  Chaos was a state of random disorder existing in primordial emptiness, a void that was created when heaven separated from earth and she had no form being an abyss.  Eventually all the elements consolidated resolving themselves into two widely different substances, a lighter portion which soared above and formed the sky and became known as Caelus or Father Sky and the solid mass beneath, which became known as Terra or Mother Earth.  Caelus and Terra had eighteen dark children, but it only takes a little bit of light to dispel a lot of darkness and light is always able to conquer the darkness, but no matter how fast light travels, it finds that darkness got there first.

Light is a wave, and in the 1600s Christiaan Huygens, a Dutch physicist, showed that light behaves like a wave.  Light expands and radiates in all directions and it has the ability to bend around corners, as proved by Thomas Young when he conducted his double slit experiment.  In 1801, Thomas Young an English polymath and physician devised and performed an experiment to measure the wavelength of light.  The difficulty confronting Young was that the usual light sources of the day (candles, lanterns, etc.) could not serve as coherent light sources.  Young’s method involved using sunlight that entered the room through a pinhole in a window shutter.  A mirror was used to direct the pinhole beam horizontally across the room.  To obtain two sources of light, Young used a small paper card to break the single pinhole beam into two beams, with part of the beam passing by the left side of the card and part of the beam passing by the right side of the card.  Since these two beams emerged from the same source, the Sun, they could be considered coming from two coherent sources.  Light waves from these two sources (the left side and the right side of the card) would interfere.  The interference pattern was then projected onto a screen where measurements could be made to determine the wavelength of light.  Young’s double slit experiment convinced most physicists that light is a wave, not a particle.

Einstein thought that light was bent by gravity and when this was confirmed, it gave credence to his Theory of general Relativity.  His theory could only be tested when there was a total solar eclipse, by observing fixed stars when the moon lined up to block the sun.  During this eclipse, the Sun would sit in front of the Hyades, a cluster of bright stars in the constellation of Taurus.  Thus, at totality, many stars would be visible near the eclipsed disk and their positions relative to the Sun could be recorded and measured on photographic plates, and then compared with reference plates showing the stars when the Sun was nowhere near the field of view.

The photoelectric effect was discovered in 1887 by the German physicist Heinrich Rudolf Hertz.  In connection with work on radio waves, Hertz observed that, when ultraviolet light shines on two metal electrodes with a voltage applied across them, the light changes the voltage at which sparking takes place.  Under the right circumstances light can be used to push electrons, freeing them from the surface of a solid.  Max Planck invented the notion of quantized electromagnetic radiation as a way to solve a technical problem with idealized sources of electromagnetic radiation called blackbodies which evolved out of predictions that suggested that an ideal black body at thermal equilibrium would emit energy proportional to the frequency squared.  Thus, the amount of energy radiated would get greater and greater at higher and higher frequencies, which conflicted with the law of conservation of energy.  Planck did not believe that radiation was actually broken up into little bits but he was able to make mathematical calculations that were based on this.  He used this theory as a contrivance to give him the right answers to resolve the Ultraviolet Catastrophe which described this failure.

Albert Einstein recognized that Planck’s idea fit in with reality, because even though we perceive light as a continuous wave of electromagnetic radiation, it is actually a stream of discrete particles, which he called photons.  In `1905, Einstein explained the photoelectric effect, by proposing the existence of discrete energy packets during the transmission of light, creating the quantum theory of light.

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