Rainbows, as well as peacock feathers are a results of
optical phenomenon, called scattering. Figuring out how those light effects behind
the scene are happening may rob you of their beauty. Let’s investigate that
little phenomenon which beautifies the world.
Children often ask questions about the world with all
their naivety. We as adults sometime realize that we don’t have the answer to
those questions. We may realize that we never asked those questions ourselves.
Some topics come more often than others: “Why is the sky blue?” or “How are
The answer to those two questions is scattering. We
also may answer that it’s mother nature’s magic. But she’s cool and she left us
several evidences to better understand her tricks.
Two types of light scattering
Scattering occurs when light rays collide with a
particle and this particle diverts the ray. If the ray is redirected in
specific direction, it’s called anisotropy. Imagine Sasuke (the one from manga
Naruto) modeling his chakra to create the one thousand bird technique, the
emitted energy goes in random directions with shape of lightning.
Figure 1: figure of anisotropic of a
particle, illustration of the character Sasuke and his technic of one thousand
If the ray is redirected in every direction without
difference, the we call it isotropy, as a sphere or a cupola. In manga world we
can compare it with version 1 of Naruto’s Rasengan technic.
Figure 2: Figure of isotropic
diffusion of a particle, illustration of rasengan technic of Naruto.
Scattering happens with all kind of waves. For
simplifying it’s like playing to the game of broken telephone. The main
information reaches someone (incoming beam), and when this person gets it, they
need to spread the information to people around them. They may say it equally
to people around them without preference (isotropic scattering), or they may
also spread the information to only a few members of the group, with
preferences (anisotropic scattering). In both cases the information has the
Tyndall effect an example of disco ball
The Tyndall effect is more or less like a disco ball
(the one from night clubs). Light shines on it, and it scatters the rays in all
directions. Those scattered rays can form a shape as a halo around the ball
(isotropic diffusion) or just in one principal direction (anisotropic
diffusion). However, particles need to have a specific size to enable this
Illustration figure of Tyndall effect
Figure 4: Illustration photo of
If touched particles have a size equivalent to
hundredth of a hair diameter (10nm), close to the wave length of the incoming
wave, they can produce this effect. Not all particles behave the same.
When a particle is in touch with light
(electromagnetic radiation), the electron cloud of the particle warp itself.
The core is no more in the center of the particle. It is like observing a child
Hula hoop from the sky. When the hoop (electron cloud) falls to the ground the
kid is in the center of the hoop. As soon as they start, they aren’t in the
Figure 4: Figure
of kid on hip pity-hop observed from the above
The electron cloud does oscillate on the same
frequency as the incoming beam. This oscillation generates a beam.
According to a British physicist, Lord Rayleigh: the
power of emitted beam is conversely proportional to the wavelength of the
incoming beam. The shorter (blue/purple wavelength) the wavelength, the more
powerful is the diffused beam. If you irradiate natural light (sunlight) on
disco balls, and if those disco balls have a size approximatively equal to a
hundredth of a hair size, they will emit a light. The blue part of light will
be the most powerful.
Most typical example is the blue sky. Now you know why
the sky is blue. there is a lot of small particles in the atmosphere. When they
get irradiated with a sun beam they diffuse it, but mostly the blues and purples.
Particles will emit blue/purple/green more powerful than others.
The same principal applies to people with blue eyes.
Figure 5: Figure of Rayleigh
In our eyes there is stroma and pigment epithelium.
They contain melanin and lipofuscin which are pigments.
Figure 6: Figure of cornea presented
as multi layers systems
Those pigments are particles which react with the
incoming light and diffuse it, when light hits the eyes. This light will give
the blue/green color that we see in peoples’ eyes.
Rayleigh diffusion and Tyndall effects are particular
forms of a larger theory, called Mie theory. This theory applies to particles
with a size between 0.1 and 10 times the of a certain wavelength. If we say
that wavelength is equal to average human height (1m70) then particles sizes
are between a little rat size (17cm) and two adult anacondas (17m).
Several specific cases of this theory explain natural
phenomena like rainbows or colors of peacock feathers. Lorrenz Mie theory
enables the use of granulometry, which is the science of measuring particle
diameters. Mastering particles sizes may be of great use in medicine production
processes of making substances such as powdered or freeze-dried. Particle size monitoring
can be done during grinding process thanks to granulometry with laser. Monitoring and adjusting particles size impact
the efficiency of medicines.
Scattering has a lot of application fields, as radars
and sonars (for backscattering of sound waves). The phenomenon which enables a
car to reverse without crashing into another is the same that create rainbows.
Understanding this demystifies one part of nature but makes natural phenomenon even
between geometrical optics and Lorenz-Mie theory
5. Generalized Lorenz–Mie theory and