Light scattering by particles in the context of Light scattering

In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions are much larger than the wavelengths of the photons in question), the photons can be said to follow Snell's law. Translucency (also called translucence or translucidity) is the physical property of allowing light to pass through the material (with or without scattering of light). It allows light to pass through but the light does not necessarily follow Snell's law on the macroscopic scale; the photons may be scattered at either of the two interfaces, or internally, where there is a change in the index of refraction. In other words, a translucent material is made up of components with different indices of refraction, while a transparent material is made up of components with a uniform index of refraction. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color. The opposite property of translucency is opacity. Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesia in an order system with three variables, including transparency, translucency and opacity among the involved aspects.

When light encounters a material, it can interact with it in several different ways. These interactions depend on the wavelength of the light and the nature of the material. Photons interact with an object by some combination of reflection, absorption and transmission.Some materials, such as plate glass and clean water, transmit much of the light that falls on them and reflect little of it; such materials are called optically transparent. Many liquids and aqueous solutions are highly transparent. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are mostly responsible for excellent optical transmission.

Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically perceived by humans as warmth) and ultraviolet (which can have physiological effects such as sunburn) lights. However, according to the American Meteorological Society, there are "conflicting conventions as to whether all three [...] are referred to as light, or whether that term should only be applied to the visible portion of the spectrum". Upon reaching the Earth, sunlight is scattered and filtered through the Earth's atmosphere as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat (atmospheric). When blocked by clouds or reflected off other objects, sunlight is diffused. Sources estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day; this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance.

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D₃ synthesis and a mutagen.

The Tyndall effect is light scattering by particles in a colloid such as a very fine suspension (a sol). Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, so blue light is scattered much more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motorcycles, in particular two-stroke machines where the burnt engine oil provides these particles. The same effect can also be observed with tobacco smoke whose fine particles also preferentially scatter blue light.

Under the Tyndall effect, the longer wavelengths are transmitted more, while the shorter wavelengths are more diffusely reflected via scattering. The Tyndall effect is seen when light-scattering particulate matter is dispersed in an otherwise light-transmitting medium, where the diameter of an individual particle is in the range of roughly 40 to 900 nm, i.e. somewhat below or near the wavelengths of visible light (400–750 nm).

Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in Earth's atmosphere, when the centre of the Sun's disc has reached 18° below the observer's horizon. This morning twilight period will last until sunrise (when the Sun's upper limb breaks the horizon), when direct sunlight outshines the diffused light.

The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System. This system of particles has been studied for many years in order to understand its nature, origin, and relationship to larger bodies. There are several methods to obtain space dust measurement.

In the Solar System, interplanetary dust particles have a role in scattering sunlight and in emitting thermal radiation, which is the most prominent feature of the night sky's radiation, with wavelengths ranging 5–50 μm. The particle sizes of grains characterizing the infrared emission near Earth's orbit typically range 10–100 μm. Microscopic impact craters on lunar rocks returned by the Apollo Program revealed the size distribution of cosmic dust particles bombarding the lunar surface. The ’’Grün’’ distribution of interplanetary dust at 1 AU, describes the flux of cosmic dust from nm to mm sizes at 1 AU.

The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in a roughly triangular shape along the zodiac, and appears with less intensity and visibility along the whole ecliptic as the zodiacal band. Zodiacal light spans the entire sky and contributes to the natural light of a clear and moonless night sky. A related phenomenon is gegenschein (or counterglow), sunlight backscattered from the interplanetary dust, which appears directly opposite to the Sun as a faint but slightly brighter oval glow.

Zodiacal light contributes to the natural light of the sky, though since zodiacal light is very faint, it is often outshone and rendered invisible by moonlight or light pollution.

A sunbeam, in meteorological optics, is a beam of sunlight that appears to radiate from the position of the Sun. Shining through openings in clouds or between other objects such as mountains and buildings, these beams of particle-scattered sunlight are essentially parallel shafts separated by darker shadowed volumes. Their apparent convergence in the sky is a visual illusion from linear perspective. The same illusion causes the apparent convergence of parallel lines on a long straight road or hallway at a distant vanishing point. The scattering particles that make sunlight visible may be air molecules or particulates.

Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to luminance arising from artificial lighting, skyglow may also involve any scattered light seen at night, including natural ones like starlight, zodiacal light, and airglow.

In the context of light pollution, skyglow arises from the use of artificial light sources, including electrical (or rarely gas) lighting used for illumination and advertisement and from gas flares. Light propagating into the atmosphere directly from upward-directed or incompletely shielded sources, or after reflection from the ground or other surfaces, is partially scattered back toward the ground, producing a diffuse glow that is visible from great distances. Skyglow from artificial lights is most often noticed as a glowing dome of light over cities and towns, yet is pervasive throughout the developed world.

Crepuscular rays, sometimes colloquially referred to as god rays, are sunbeams that originate when the Sun appears to be just above or below a layer of clouds, during the twilight period. Crepuscular rays are noticeable when the contrast between light and dark is most obvious. Crepuscular comes from the Latin word crepusculum, meaning "twilight". Crepuscular rays usually appear orange because the path through the atmosphere at dawn and dusk passes through up to 40 times as much air as rays from a high Sun at noon. Particles in the air scatter short-wavelength light (blue and green) through Rayleigh scattering much more strongly than longer-wavelength yellow and red light.

Crepuscular rays appear as divergent beams emanating from a distant source, in spite of the rays from the Sun being parallel when they arrive, because of perspective. The point from which the divergent rays appear to emerge from is really a vanishing point for parallel rays of sunlight.

The Belt of Venus, also called Venus's Girdle, the antitwilight arch, or antitwilight, is an atmospheric phenomenon visible shortly before sunrise or after sunset, during civil twilight. It is a pinkish glow that surrounds the observer, extending roughly 10–20° above the horizon. It appears opposite to the afterglow, which it also reflects.

In a way, the Belt of Venus is actually alpenglow visible near the horizon during twilight, above the antisolar point. Like alpenglow, the backscatter of reddened sunlight also creates the Belt of Venus. Though unlike alpenglow, the sunlight scattered by fine particulates that cause the rosy arch of the Belt shines high in the atmosphere and lasts for a while after sunset or before sunrise.

Forward scattering is the deflection of waves by small angles so that they continue to move in close to the same direction as before the scattering. It can occur with all types of waves, for instance light, ultraviolet radiation, X-rays as well as matter waves such as electrons, neutrons and even water waves. It can be due to diffraction, refraction, and low angle reflection. It almost always occurs when the wavelength of the radiation used is small relative to the features which lead to the scattering. Forward scatter is essentially the reverse of backscatter.

Many different examples exist, and there are very large fields where forward scattering dominates, in particular for electron diffraction and electron microscopy, X-ray diffraction and neutron diffraction. In these the relevant waves are transmitted through the samples. One case where there is forward scattering in a reflection geometry is reflection high-energy electron diffraction.