Nature of Light
Light is a form of electromagnetic energy emitted from a point source, and spreading as a wave front in all directions, in a spherical form.
The traveling speed of this front is the speed of light, which is 300,000 Km per second in vacuum, and its slowed by 25% when travelling through water, and 33% when through glass. This wave front can be diverted from its original direction by objects along its path. The nature of these objects dictates the level of diversion. The behavior of electromagnetic energy is better explained and understood if we assumed that they act like waves. To visualize this, think of a stone dropped into a pond. We can see circular rings of water advancing along the surface. Electromagnetic waves are traveling the same way, but in three dimensional (spherical) manner. The distance between one crest of wave to the next is called the wavelength. The number of wave crests passing a specific point at a measurable time frame is the frequency. The height, or the intensity of the wave is called amplitude.
Different electromagnetic energy sources are emitting various frequencies or wavelengths. The range of these various types of wavelength is called the electromagnetic spectrum . At the lower end, we find the radio waves, and while the wavelengths are getting shorter they appear as heat, then as infra red, the visible spectrum, ultra violet, nearly at the end of this range we can detect what is called x-rays and gamma rays. There is no definite gap when one group merges with another because the change is continuous along the whole spectrum.
The visible spectrum is the portion of the electromagnetic range that stimulates the human eye. The range is between 400 nanometer (wavelength) and 700 nanometer. With red at the lower part (700 to 600 nanometer) blue and the violet at the higher end (400 nanometer.). At the fringes of this range we can find the infra red light ( 720 nanometer) and the ultra violet ( 390 nanometer). Again, there are no breaking points, and the changes are continuous.
Spectral Quality is the combination of wavelengths, with different energy levels, of a given light source. Imagine preparing cocktails - we can create endless combinations of different aromas and liqueurs. Every light source is a cocktail of various wavelengths, each wavelength has different intensity.
As mentioned earlier, light can be diverted from its path. When light hits an object, the object may absorb specific segments of light and reflect other portions. These reflections when reaching the human eye are translated as the colors of an object. If we use different light sources with a different spectral quality, the color of the same object may appear to us slightly different every time. Daylight is one of the most common sources of light in everyday life. But is it the same all the time everywhere under the sun? The answer is definitely not. As it leaves the sun it filters through space and the atmosphere, both ever changing environments. It hits the earth surface in different angles according to the time of the day and the season of the year. Changing weather conditions, pollution levels, reflecting land textures: all these dynamic elements are the reason why the spectral quality of daylight is very illusory.
Artificial light sources have developed in leaps and bounds since Edison invented the light bulb. Todays manufacturers are producing an ever-growing range of light emitting techniques. Therefore a large but not necessarily standardized range of light sources are available. We can expect steady spectral quality from any given type of artificial source. But the large variety makes it impossible to set one type as the international standard. Also, the importance of spectral quality is limited to specific fields: one of them is the gemstone trade.
Finally, would a same object, a gemstone for instance, under an early morning Bangkok sun, look slightly different then it would from under an Arizonian afternoon sky? To a certain extend, yes. Some types of gemstones will demonstrate greater color change then others, and more often then not, these differences will affect the price of a stone. Usually gem dealers would inspect gemstones in bright (but not direct) daylight. It is however good practice to avoid buying stones under cloudy sky. A set of reference stones with full color range can be an essential tool to be used under any light. A trained eye can distinguish small variants when comparing a new stone to a reference piece. The most important thing is to be aware of the variable nature of light and the magical way it colors our world.