Lesson 12

# Lesson 12 - Lesson 12 Colour and Light Learning Objectives...

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Lesson 12 - Colour and Light Learning Objectives By the end of this Lesson, learners will: Understand the concept of electromagnetic radiation (light) Understand the relationship between colour and light Describe the effects of illumination type on colour Distinguish between the reflection and refraction of light Understand the concepts of refractive index, isotropy, and anisotropy Understand the principle of dispersion and how it relates to "colour" Describe the three main ways to generate colour in gems (allochromatic, pseudochromatic, idiochromatic) Describe the different degrees of transparency Introduction A large part of the beauty and value of gemstones and precious metals revolves around the interaction between light and the object. This includes not only the hue and saturation of colour, but also how light is transmitted, reflected, refracted, fluoresced, and dispersed. Light is electromagnetic radiation or energy, and can be described as behaving like both waves and particles (photon). Like all waves, light can be described by its wavelength, the distance from peak to peak or trough to trough, and its frequency, the number of wave crests (or troughs) that pass through one point in one second. Light propagates in the direction of its wave front. All electromagnetic radiation (from radio waves to x rays) travels at a constant speed. So, when the frequency of light is decreased, its wavelength must increase this is an inverse relationship. Light energy increases with increasing frequency (or decreasing wavelength). Refer to the figure and table below for more information on the parts of a wave. Light also behaves like a particle when it travels as photon particles. More intense light would be composed of a greater number of photons with a higher frequency of incidence. For gemstones, interaction with light is best described using the wave-like approach. For those students interested in the wave-particle duality of light, a good online starting point is

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Wikipedia . Anatomy of a wave . Parameter Symbol Definition (units) crest -- highest point of a wave trough -- lowest point of a wave wavelength λ distance between two successive crests or two successive troughs of a wave (nanometres or 10 -9 metres, nm) frequency f number of waves passing a point per unit of time (cycles per second or hertz, Hz) amplitude A vertical distance between crest or trough and the equilibrium line (nanometres or 10 -9 metres, nm) Extra Readings Nassau, K. (1978) " The Origin of Color in Minerals ", American Mineralogist , Vol. 63, p219-229. Extra online resources: Causes of Color , from the Institute for Dynamic Educational Advancement (IDEA). What is Colour? Colour is what our brain interprets from the incidence of light (electromagnetic radiation within the visible spectrum) on our eye. In other words, the colour of an object is our eye's interpretation of light in the visible range that has interacted with the object we are looking
at.

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