Nature of light

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The beauty of a gemstone depends almost entirely upon the way it effects the light. The fire of a Diamond and the play of colors in an Opal are just two examples.

Scientists recognize two different theories to explain the way in which light is transmitted:

  • The electromagnetic wave theory
  • The quantum theory

Both theories are important to learn so you may understand the causes of color in gemstones.

The electromagnetic wave theory

Basic

This theory was developed by Dutch scientist Christian Huygens and later explored by others.

Light is a form of energy traveling in a wave motion.
It can be compared to what happens when you throw a brick in a puddle of water: the water's surface starts to vibrate and these vibrations cause circular waves.

Fig.1: Transversal light


The wave motions of light are not circular but spherical.
We can't see the light because:

  • It travels too fast. The velocity of light (in air) is approx. 300,000 km/second.
  • The individual waves are too small.

Note: although we can't see light we can see color. This is caused by absorption.

The light waves are transversal . This means that light travels in all directions in a straight line from its source.


Fig.2: Wavelength and amplitude

There is a relationship between wavelength and color of light which is explained by what we call the electromagnetic spectrum. Red light travels at a wavelength between 740 and 625nm, blue light between approx. 420 and 380nm. Wavelength is measured in nanometers (nm), which is one-billionth of a meter.
An older measurement unit is Ångström, which is 10nm but this unit is no longer used.

The amplitude of a wave defines the intensity of the color. The higher the amplitude, the more intense the color. If there is almost no amplitude in the wave, then the color will be almost to entirely black.


The quantum theory

Basic

This theory was developed by Planck and Einstein.

It states that light energy can be absorbed and emitted only in small and discrete amounts, called "quanta" (later known as "photons"). This theory is used to explain certain causes of color in gemstones and the phenomenon of fluorescence.
Fluorescence is the emission of visible light by a substance, such as a gemstone, when irradiated by energy of a shorter wavelength. Some rubies can be made to glow like red-hot coals when they are bombarded by ultra-violet radiations.

Fig.3: Photons (light energy) traveling in small packages ("quanta")

This theory is very important when explaining the causes of color in gemstones, which is primary due to absorption of energy (from the photons) and the emitting of that energy.

The electromagnetic spectrum

Basic

Fig.4: The electromagnetic spectrum


The electromagnetic spectrum consists of the entire range of wavelengths, from the longest radio waves through continually shortening wavelengths, to infra-red, visible light, ultra-violet and X-rays. And finally the shortest waves of all, cosmic rays.

The seven colors of the visible light spectrum are: Red, Orange, Yellow, Green, Blue, Indigo and Violet.

When all these colors are mixed, we see what we call "white light". And when there is no color at all, we see black.
Infra-red is used in reflectivity meters, as an identification aid.
Visible light reveals the color and beauty of gemstones.
Ultra-violet may produce fluorescence effects.
X-rays may also produce fluorescence, it may be used for color alternation and enable us to distinguish between various types of pearls.
Gamma rays may be used to alter the color of certain gemstones.