Introduction

With regards to the absorption of light, primary material considerations include:

• At the electronic level, absorption in the ultraviolet and visible (UV-Vis) portions of the spectrum depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific frequency, and also does not violate selection rules. For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules. Thus, there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
• At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and also on selection rules. Nitrogen and oxygen are not greenhouse gases because the absorption is forbidden by the lack of a molecular dipole moment.

With regards to the scattering of light, the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include:

• Crystalline structure: How close-packed its atoms or molecules are, and whether or not the atoms or molecules exhibit the long-range order evidenced in crystalline solids.
• Glassy structure: Scattering centers include fluctuations in density and/or composition.
• Microstructure: Scattering centers include internal surfaces such as grains, grain boundaries, and microscopic pores.

Nature of light

Complete spectrum of electromagnetic radiation with the visible portion highlighted

The spectrum of colors which collectively constitute white (or visible) light, as seen in their dispersion through a triangular dispersive prism

Radiant energy is energy which is propagated in the form of Electromagnetic waves. The type of light which we perceive through our optical sensors (eyes) is referred to as white light, and it is composed of a range of colors (ROYGB: red, orange, yellow, green, blue) over a range of wavelengths, or frequencies. Visible (white) light is only a small fraction of the entire spectrum of electromagnetic radiation. At the short end of that wavelength scale is invisible ultraviolet (UV) light. At even shorter wavelengths than UV are X-rays and gamma-rays. At the longer end of that spectrum is infrared (IR) light, which is used for night vision and other heat-seeking devices. At longer wavelengths than infrared are microwaves (radar), and radio / television waves.

Electromagnetic radiation is classified according to the frequency (or wavelength, which is inversely proportional to the frequency) of the light. This includes (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet (UV) radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. A small window of frequencies, called the visible (or white light) portion of the spectrum, is sensed by the naked eye of various organisms.^[2]

The simplest representation of a beam of light is through the use of the light ray. The most important properties of the light ray are that it contains no mass and that it travels along a straight line. Light rays interact with the materials (liquids and solids) in several different ways; it is absorbed, reflected or transmitted by the object. In the case of reflection, the interaction depends on the physical and chemical properties of the substance. If the materials surface is perfectly smooth (e.g. a mirror), rays of light collectively undergo total reflection (or specular reflection), leaving the surface of the glass at a particular angle and all in a parallel line with each other.

Light scattering

Main article: Light scattering

Diffuse reflection

Rough and irregular surfaces cause light rays to be reflected in many random directions. This type of reflection is called “diffuse reflection”, and is typically characterized by wide variety of reflection angles. Most of the objects visible to the naked eye are identified via diffuse reflection. Another term commonly used for this type of reflection is “light scattering”. Light scattering from the surfaces of objects is our primary mechanism of physical observation.^[3][4]

Light scattering in liquids and solids therefore depends on the wavelength of the light being scattered. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension (or spatial scale) of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometer (one millionth of a meter) scattering centers (or particles) as small as one micrometer have been observed directly in the light microscope (e.g. Brownian motion).