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5.2: Light and the Properties of Light

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    Figure 5.6: The spectrum of electromagnetic radiation

    Before starting a discussion of optical mineralogy, it is helpful to take a closer look at light and its properties. Light is one form of electromagnetic radiation (Figure 5.6). Radio waves, ultraviolet light, and X-rays are other forms of electromagnetic radiation. All consist of propagating (moving through space) electric and magnetic waves (hence the term electromagnetic). The interactions between electric waves and crystals are normally much stronger than the interactions between magnetic waves and crystals (unless the crystals are magnetic). Consequently, this book only discusses the electrical waves of light. In principle, however, much of the discussion applies to the magnetic waves as well.

    Because light waves have wavelengths two orders of magnitude greater than atom sizes or bond lengths, the interactions of light with crystals does not reveal information about individual atoms in a crystal. To obtain that kind of information, we must study crystals using shorter wavelength radiation, X-rays (Chapter 12).

    Figure 5.7: The characteristics of a wave

    Light waves, like all electromagnetic radiation, are characterized by a wavelength, λ, and a frequency, ν (Figure 5.7). The velocity, v, of the wave is the product of λ and ν:
    blankv = λν

    In a vacuum, light velocity is 3 x 108 meters per second. Velocity is slightly less when passing through air, and can be much less when passing through crystals. When the velocity of light is altered as it passes from one medium (for example, air) to another (perhaps a mineral), the wavelength changes, but the frequency remains the same.

    Figure 5.8: Different wavelengths and colors of visible light

    Visible light has wavelengths of 390 to 770 nm, which is equivalent to 3,900 to 7,700 Å, or 10-6.1 to 10-6.4 m. Different wavelengths correspond to different colors (Figure 5.8). The shortest wavelengths (violet light) grade into invisible ultraviolet radiation. The longest wavelengths, corresponding to red light, grade into invisible infrared radiation.

    Light composed of multiple wavelengths appears as one color to the human eye. If wavelengths corresponding to all the primary colors are present with equal intensities, the light appears white. White light is said to be polychromatic (many colored), because it contains a range, or spectrum, of wavelengths. Polychromatic light can be separated into different wavelengths in many ways. When one wavelength is isolated, the light is monochromatic (single colored).

    This page titled 5.2: Light and the Properties of Light is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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