LIGHT AND COLOR

Understanding Light and Color
Understanding how to control light and color requires understanding the light spectrum. Light is how we describe a form of electromagnetic energy that interacts with chemicals in the retina and generates an electrical impulse, giving rise to complex neural signals ending with the perception that we term sight. The amount of electromagnetic energy that is detected and viewable through the material is defined as the transmission of the glass.

Light is a portion of the electromagnetic spectrum. For glass purposes, we usually define light as that portion of the spectrum from the ultraviolet range through the visible range and then through the infrared range. Our eyes permit the discrimination of many individual wavelengths or combinations of wavelengths we call colors.

See the light spectrum below.

SEQUENCE OF VISIBLE RANGE OF THE ELECTROMAGNETIC SPECTRUM

When light encounters elements in the glass that are small relative to the light wavelengths, then the shorter wavelengths are scattered much more than the longer. This is the reason that a cloudless sky is blue and the overhead sun is yellow; the blue light from the sun is scattered by air molecules in the atmosphere much more than light at the red/yellow end of the spectrum. At sunset the sun's rays must traverse a longer path through the atmosphere to an observer than earlier in the day so that only the relatively non-scattered red rays will reach an observer and explains the red colors of sunset. This is known as Rayleigh scattering. Further, without an atmosphere the "sky" would be black in daytime.

When the major part of the light is transmitted through the glass without scattering or absorption, the substance is said to be transparent. When most of the light entering glass is absorbed within it, the glass is said to be opaque. In some glasses, an electron in the glass may absorb a photon of one particular wavelength, and release and transmit another longer wavelength and lower energy photon; this is called fluorescence. Note that selective absorption of particular wavelengths gives rise to the transparency and colors seen in many precious gems or colored glass.

CONTROLLING LIGHT AND COLOR WITH COATINGS AND FILTERS
Controlling light and colors with coatings and filters interact with the characteristics of reflection, refraction, and interference of the light wave passing through glass. In the section of Optical Properties of Glass, we learned reflection is when light bounces back from the surface of the glass; refraction is where light bends when passing through the glass. Another result from light passing through glass is called diffraction, which is the deviation of light when the wavelength encounters and edge or aperture.

The principle of interference is employed to reduce reflections, particularly in optical instruments and displays. This is particularly important with where much of the incident light would otherwise be lost to reflection or doubly reflected reducing image contrast, reducing overall light transmission or causing unwanted distracting images.

The coatings listed below are among the variety of ways to control how and the quantity of light passing through glass. Multi-layer coatings are utilized in high quality optical components to minimize reflections across the spectrum. Polarizing materials can also be applied to reduce image-degrading glare, such as on sunglasses or applications like ATM or computer screens. Glare-reducing filters cause light traveling through the filter to be cancelled when reflected off the surface of the screen on its return to the eye