This course has five lessons.
In this lesson we learn basics concepts followed in optical
simulation, ray propagation, simulation, and visual ergonomy.
This is our outline.
We will introduce specular reflection and its properties. Refraction and
its properties, dispersion, transmittance, absorption surface scattering volume, scattering, and
coatings.
First, let's understand the light propagation concept.
We know in a homogeneous medium light travels in a
straight line.
For example, when light travels in a vacuum, it travels
in a straight line without loss.
In any medium, light will interact with it.
As long as the medium is isotropic, light will continue
to travel in a straight line, but more or less
energy will be absorbed in the path.
When light encounters the surface of two different materials, different
phenomena occur.
Reflection the light remains in the same material or transmission.
The light enters the second material with or without diffusion,
with or without absorption.
We will now review all these phenomena.
The simplest is a specular reflection. That is no scattering.
It is pure reflection on the polished metal surface.
In fact, this is true on all polished surfaces.
The angle of incident is the angle between the surface
normal and the direction of the incident light.
The plane formed by the light rays and the surface
normal is called the incident lane.
The light reflects from the surface at an angle equal
to the angle of incident while remaining in the plane
of incidence.
The refraction of light is caused by the different speeds
of light in different media.
The greater the density of the medium, the slower the
light propagation speed.
Fair Mark clarified the principle of light propagation under the
knowledge of the principle of least action proved by more
peteus and LaGrange and expanded afterward.
Farmer's principle of lease time. The light always chooses the
smallest light path between two points.
The optical path is the distance multiplied by the coefficient
related to the medium density or optical index, which we
will see later.
Therefore we can see from the picture that the pencil
seems to be bent.
The Snell Descartes Law is a law that describes the
direction of refracted light in the phenomenon of light refraction.
From a geometric point of view, this can be determined
by the following equation to determine the direction of refracted
light.
Product of N1 and Sine thera 1 is equal to the product
of N2 and Sine theta two.
This equation takes into account the direction taken by the
light, whether it is reflection or refraction.
This equation is called Descartes in France and Snell in
England.
Two physicists seem to express it independently at the same
time.
But it must be remembered that Persian physicists approached its
sixth centuries ago.
In order to optically characterize the optical density of a
medium, we define a quantity. The refractive index, which is
equal to C, that is, the speed of light in
a vacuum divided by the speed of light in the
medium.
The refractive index is one for vacuum by definition, and
greater than one for any other medium.
Most transparent materials in the visible range have an index
between 13 and 17.
Diamond is the transparent medium with the highest refractive index.
The refractive index is actually not constant and depends on
the wavelength.
This value decreases as the wavelength increases.
For optical materials, the change in the refractive index of
the material at different wavelengths is called the convergence or
Abbey number of the material.
This change is calculated based on the refractive index measured
at three different wavelengths where.
ND is the refractive index at 589.3 nanometers.
NF is the refractive index at 486.1 nanometers.
NCC is the refractive index at 656.3 nanometers.
The refractive index is also affected by temperature and pressure,
such as gas.
This animation shows how light is deflected according to the
refractive index of the second medium.
This is Snell Descartes law.
Here the first medium is air.
In the opposite case, the light first encounters the surface
on the other side of the air in a medium
with a refractive index greater than one.
This phenomenon initially appears to be the same, but we
observe the existence of a limiting angle.
As far as the equation is concerned, this angle corresponds
to sin Theta one, which is mathematically impossible.
We will see below the physical phenomenon when the incident
angle is greater than the limit angle.
The refractive index is not a constant.
This means that the Abbey number is less than Infinity.
The refractive index always decreases as the wavelength increases. This
means that the Abbey number is a positive value.
The lower the Abbey number, the greater the index change.
Therefore, the direction of light refracted is related to the
wavelength.
Result a white beam consisting of several wavelengths will be
dispersed into multiple rays.
The most famous example of this phenomenon is the optical
effect of a prism.
Another effect related to refraction is The Mirage effect.
The refractive index depends on the density of the material.
However, for gas, the hotter it is, the lower its
density.
Therefore, the hotter the gas, the lower its refractive index.
This change is very small, but it will have a
significant impact on a long propagation distance, especially when the
atmosphere is stable and has a vertical temperature gradient.
There are two kinds of mirages at this time.
One is called the inferior Mirage, which mainly occurs on
land.
This is the case when the sun heats the surface
of the earth.
The light is deflected gradually so when you look at
the ground the light you see actually comes from the
sky.
And our feeling is the reflection on the water.
The Mirage in the Oasis in the desert and the
phenomenon we found on the road in the summer belonged
to the young Mirage.
The other is called the superior Mirage, which mainly occurs
at sea.
At sea, the air near the sea surface is colder
and the air far away from the sea surface is
hotter.
Then create a temperature gradient opposite to the previous one.
The result is to see the element in the water
in the sky.
In this way, ships all lighthouses can sometimes be seen
from a long distance.
If we go back to the Snell Descartes equation, we
will see a mathematical limit.
If the refractive index N one is greater than N2,
we can get a sign greater than one.
This is mathematically impossible.
If and one is greater than two, there is an
angle beyond which there is no Theta 2.
The light is totally reflected.
There is no transmitted light.
This phenomenon is called total internal reflection.
This is the method used in the design of many
optical devices.
The great advantage of this reflection is that it is
lossless. Unlike metal reflection are almost equal to 80%.
Please note that as we have just seen, total internal
reflection is only possible under certain conditions.
Light enters from a medium with a high refractive index
to a medium with a low refractive index.
The incident angle must be greater than the critical angle.
For example, for a material with a refractive index of
1.5, the critical angle is about 42 degrees.
When light travels through the vacuum, it does not lose
energy.
In any other medium, light interacts with matter.
These multiple interactions primarily absorb energy and photons.
From the most energetic UV to the least, energetic, MWM
we encounter the following phenomena.
Irritation.
Extracting an electron from a molecule.
Fluorescent phosphorescent?
Electrons absorb photons and jump to a high energy state,
but are not stable and electrons release photons when they
return to a low energy state.
Excitation of molecules by vibration, rotation or twisting.
Some examples.
Water absorbs infrared rays.
Vibration mechanical effect heating.
Fluorescence.
XRAY and gamma ray damage to DNA.
Direct action by breaking the chain and or indirect action
by the ionization of water molecules and the interaction of
free radicals with DNA.
From a macro optical point of view, any light material
interaction can lead to the absorption of some energy regardless
of the phenomenon involved.
The law that defines this absorption is called Lambert's law
or B's law.
This is an exponential law involving the absorption coefficient A
expressed in millimeter one, and propagation distance in millimeter.
The amount of absorption directly proportional to the propagation distance.
It also depends on the wavelength.
This absorption can be used to determine the composition of
the medium.
Absorption in the atmosphere can provide information about the presence
of molecules, pollutants.
The absorption concept is used in developing filters to project
colors in projectors.
As long as the surface of the medium is perfect,
the Snell Descartes and frenel laws apply.
Here the definition of a perfect surface is a polished
surface without roughness.
But if the surface is rough, then these formulas are
not applicable.
Here we use the macroscopic scattering model. For example, the
simplest way to divide the reflection into 3 categories.
Specular reflection, Gaussian and Lambertian.
The reflection on a given surface can be described as
a combination of its three contributions.
But be aware that this model is usually too simplistic
because the distribution depends on incident angle and wavelength.
The reflection properties of different wavelengths and different incident angles
are different.
Therefore we use the concept of BRDF.
BRDF BSDF and BSDF are functions that can simulate how
light reflects or transmits on a given surface.
This function is highly dependent on the incident of light.
It can also depend on the wavelength.
It may also depend on polarization, especially for slightly diffused
surfaces.
Finally, it can depend on the orientation of the surface.
For example, brushed aluminum.
We tend to come into contact with nonuniform material with
volume scattering function.
The bulk scattering material contains microscale particles, pigments, etc can
interact with light, such as reflection, refraction, dispersion, diffraction, etc.
And the macroscopic result is that the light is scattered.
Usually several methods can be used to model the scattering
model.
Mie theory.
Henyey, Greenstein, establish a scattering model by determining the scattering
coefficient and anisotropy factor, as well as the equation.
Another commonly used material is optical coating.
By depositing a very thin layer of material on the
surface of the material, the reflectance are and transmittance to
of the material surface is adjusted.
The coating is often a multilayer stack structure combining several
materials, usually dielectric, in a continuous layer and with a
perfectly controlled thickness to achieve the desired result.
The effect of the coating depends on the incidence, wavelength,
and polarization of the light.
The design of this filter is relatively complicated and involves
specific software and models interference optics.
Commonly used coating applications are.
Filter with pass band type, dichroic type, UV protection or
IR thermal filter.
Although they are very efficient, the cost of such devices
is often an obstacle to their use.
Thank you for following this session on basic concepts of
light.
See you in the next lesson on the definition of
light and the different types of sources commonly used.