Light: scattering

Principle

Scattering is a process whereby some forms of radiation, such as light or moving particles, are forced to deviate from a straight trajectory by non-uniformities in the medium. It occurs also with sound. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection (see Light: refraction). Reflections that undergo scattering are often called diffuse reflections and unscattered reflections are called specular (mirror-like) reflections.

The types of non-uniformities that can cause scattering, the scatterers or scattering centers are for instance particles, bubbles, droplets, cells in organisms, density fluctuations in fluids and surface roughness.

Scattering can be distinguished between two broad types, elastic and inelastic. Elastic scattering involves no change of radiation energy, but inelastic scattering does. If the radiation loses a significant proportion of its energy, the process is known as absorption. This is governed by the Lambert-Beer law.

Major forms of elastic light scattering are Rayleigh scattering and so called Mie scattering. Inelastic EM scattering effects include Brillouin scattering (see More Info).

With Rayleigh scattering of light, or EM radiation, it holds that particle diameter d < 0.1λ (λ is wavelength) of the light. It occurs when light travels in transparent solids and liquids, but especially in gases. Rayleigh scattering is proportional to λ^-4.

If d > λ, light is not separated in all its wavelengths and the scattered light appears white, as do salt and sugar.

Light scattering is one of the two major physical processes that contribute to the visible appearance of most objects. The other is absorption. Surfaces described as white owe their appearance almost completely to the scattering of light by the surface. The absence of surface scattering leads to a shiny or glossy appearance. Light scattering can also give color to some objects, usually shades of blue (as with the sky, the human iris, and the feathers of some birds), but resonant light scattering in nanoparticles can produce different highly saturated and vibrant hues.

Application

In medicine

Scattered light is the image forming light in dark field microscopy. Scattered sound plays the same role in Echography. However, in many applications of computer-generated imagery one tries to minimize the disturbance influence of scattering.

Some specific way of scattering is of importance for detecting DNA, proteins and for Fluorescence.

In ophthalmology it is of importance with respect of the quality of the eye media. Scattering in the eye media, especially in the eye lens disturb clear vision, especially under scotopic conditions. In vision research diffusers (see More Info) are often applied.

In daily life

Why is the sky blue? This effect occurs because blue photons hit the air molecules in the earth's atmosphere and are scattered down to the earth's surface. Red photons are not affected by the particles and pass on through the earth's atmosphere. This causes blue light to be scattered down to the earth's surface which makes the sky appear blue.

During sunrise and sunset the Sun's light must pass through a much greater thickness of the atmosphere to reach an observer on the ground. This extra distance causes multiple scatterings of blue light, but relatively little scattering of red light. This is seen as a pronounced red-hued sky in the direction towards the sun: an orange-red sun, which is yellow during daytime.

For the sun high overhead, sunlight goes through a much smaller atmospheric layer, so little scattering takes place. This is why the sky close to the overhead sun in midday appears mostly white, the sun's color.

More info

When radiation is only scattered by one localized scattering center, this is called single scattering, but mostly scattering centers are grouped together, and multiple scattering occurs. The main difference between the effects of single and multiple scattering is that single scattering can usually be treated as a random phenomenon and multiple scattering is usually more deterministic. Single scattering is often described by probability distributions.

With multiple scattering, the final path of the radiation appears to be a deterministic distribution of intensity as the radiation is spread out. This is exemplified by a light beam passing through thick fog. Multiple scattering is highly analogous to diffusion and the terms multiple scattering and diffusion are interchangeable in many contexts. Optical elements designed to produce multiple scattering are thus known as diffusers or Lambertian radiators or reflectors.

Not all single scattering is random. A well-controlled laser beam can be exactly positioned to scatter off a microscopic particle with a deterministic outcome. Such situations are encountered in radar scattering from e.g. a car or aircraft.

Rayleigh scattering

The inherent scattering that radiation undergoes passing through a pure gas or liquid is due to microscopic density fluctuations as the gas molecules move around.

The degree of Rayleigh scattering varies as a function of particle diameter d, λ, angle, polarization (see Light: polarization), and coherence (see Light). The intensity I of light scattered by a single small particle from a beam of unpolarized light of intensity I₀ is given by:

where l is the distance to the particle, θ is the scattering angle, n is the refractive index (see Light: refraction) of the particle.

The angular distribution of Rayleigh scattering, governed by the (1+cos²θ) term, is symmetric in the plane normal to the incident direction of the light, and so the forward scatter equals the backwards scatter.

Mie scattering

For larger diameters the shape of the scattering center becomes much more significant and the theory only applies well to spheres, spheroids (2 equal axes) and ellipsoids (3 unequal axes).

Both Mie and Rayleigh scattering of EM radiation can undergo a Doppler shift (see Doppler principle) by moving of scattering centers.

At values d/ λ > 10 the laws of geometric optics are mostly sufficient to describe the interaction of light with the particle, and at this point the interaction is not usually described as scattering.

Another special type of EM scattering is coherent backscattering. A description of this phenomenon is beyond the scope of this compendium.

Tyndall effect

This is the effect of light scattering on particles in colloid systems, such as emulsions (see Colloid and Emulsion). The effect distinguishes between these types of colloids. It is proportional to d⁶ and hardly on λ.

Brillouin scattering

This occurs when light in a medium (such as water or a crystal) interacts with density variations and changes its path. When a medium is compressed n changes and the light's path necessarily bends. The density variations may be due to acoustic modes, vibration phenomena in crystals (phonons) or temperature gradients.