The scattering of light by an electron in an atom is the phenomenon that accounts for reflection and transmission (refraction) in a medium. Light scattering, among other factors, is also responsible for the visible appearance or perception of color and may be divided into 3 regimes—Rayleigh scattering, Mie scattering, and geometric scattering—according to the dimensions of the scattering medium with respect to the wavelength of the incident light. Note that the Tyndall effect, a term commonly used in ophthalmology and chemistry for scattering of light by colloidal suspensions, does not describe a separate scattering regime, but rather the effect of the light beam becoming visible. For example, normally you cannot see a beam of light in water or the aqueous humor. Shining a flashlight beam in a glass of water after adding a few drops of milk is an excellent demonstration of the Tyndall effect. The milk proteins will stay suspended in the water (a colloid suspension), making the beam visible. Similarly, if we can see the beam of light in the aqueous humor during slit-lamp examination, this is an indication of inflammation, in other words an indication resulting from the presence of suspended protein in the aqueous.
Rayleigh scattering occurs when light interacts with particles much smaller than the wavelength of the light. The degree of this form of scattering varies strongly with wavelength, more precisely, inversely to its fourth power. Therefore, the probability that light will scatter is much higher for shorter wavelengths (higher frequencies), such as blue light, than for longer wavelengths (lower frequencies), such as red light.
The effect of Rayleigh scattering of sunlight on gas molecules that make up the Earth’s atmosphere is what produces the blue appearance of the sky during daytime and its reddish appearance during sunrise or sunset. This is because during daytime, when the sun is overhead, the highly scattered blue end of the spectrum that is emitted by the sun reaches your eye from all directions, whereas the weakly scattered redder wavelengths miss your line of sight when you look at the sky away from the direction of the sun. At sunrise and sunset, with the sun at the horizon, it is the weakly scattered red end of the spectrum that can still be seen, passing the atmosphere towards your eyes almost undeviated, while the blue light that is scattered off misses your line of sight.
The bluish appearance of the cornea and lens (particularly noticeable in young eyes) under slit-lamp examination is also due to Rayleigh scattering by stromal collagen and lenticular fiber cells.
Mie scattering is produced by particles whose size is the same order of magnitude as the wavelength of incident light. Unlike Rayleigh scattering, this form of scattering, on average, does not vary strongly with wavelength and tends to be stronger in the forward direction than in any other direction.
Mie scattering contributes to the white appearance of clouds. This is because the water droplets that make up the cloud are of comparable size to the visible wavelengths, so that the probability of being scattered is about identical for the different wavelengths (or frequencies) in the white sunlight, and the clouds therefore appear to be white.
In the eye, age-related increase in scattering is mostly associated with an increase in Mie scattering by cataract formation, which adds a strong forward, but less wavelength-dependent, component to the Rayleigh scattering (the predominant scattering component in healthy young eyes). Mie scattering caused by a cataract accounts for the whitish appearance of the lens under slit-lamp examination. The increase in forward scattering is what results in decreased contrast of the patient’s retinal image, and hence increased glare sensitivity.
If the particle size is much larger compared to the wavelength of incident light, the interaction of light with the particle is usually sufficiently described by the laws of geometric optics (refraction and reflection). For example, refraction and reflection are sufficient to explain the formation of a rainbow. This is because raindrops are larger than water droplets in clouds and can be considered as merely refracting and reflecting the white sunlight from their surfaces. This nicely illustrates how at the macroscopic level, the quantum behavior of light can be conveniently disregarded.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.