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Reflection, Refraction, IOR, TIR, Dispersion

Updated: May 20, 2022

In this article, we try to explain the interactions of light with various type of objects and the outcomes.



In our previous article, we tried to explain the forming and qualities of light and colors with a bit of physics-based theory.


Now we will dive a bit deep and try to explain some of observable qualities of light.


When light interacts with an object, the most important factor of the outcome is the structure of the object. Light shows different interactions with air, water, soil, wood etc.. Objects are studied in three form according to their light transmittance:


  • Opaque - No way light can penetrate (stone, iron etc.)

  • Transparent - Conductive. Light can penetrate. (glass, water, jellyfish, air..)

  • Translucent - Semi-conductive. It allows some amount of light to penetrate while reflects back the rest, it distorts light direction and scatters the light rays. (Paper, orange juice, plastic milk bottle, marble! )


When light encounters one of these substances, its direction changes in two basic ways.


When it hits an opaque object, the light is reflected back.


When it hits a transparent object, it passes through it with a slight or heavy directional deviation which is called "refraction". Index of Refraction of a transparent object is the most important factor over the directional deviation of light. We will talk about it in Refraction section.


When the light hits a translucent object, it penetrates to some degree but by the entering in a medium more dense than a transparent object, light scatters a lot inside the object, even the frequencies can be diverged.


Depending on the thickness of the object, light will manage to escape from less dense areas while leaving most of its energy behind.



Reflection

In core, reflection is the refraction. Basically, any change in direction of light is refraction.

If perpendicular light hits an object and the direction changes at an angle of 90 degrees or more, or direction of light (disregarding angle of light source) changes at any angle without penetrating surface it is called "reflection".


Reflection is also studied under two topics as Diffuse and Specular.


Diffuse Reflection


Light rays travel linearly. If there are micro, even atomic scale irregularities and roughnesses on the surface, the light rays are highly reflected between these irregularities and the integrity of rays will be disrupted and scattered in form of diffusion.


Stone, wood, metal etc...





Specular Reflection


There is no perfectly smooth object. If an object is smoothed enough, incoming light rays reflects back linearly as they came without scattering, without losing its integrity and energy.

Processed objects such as mirror, glass, aluminum-chrome are some examples.

The only natural substance is the water. Due to its viscosity and molecular structure, water always tends to be a smooth, perfect surface.




Let's explain these two types of reflection with an example from everyday life. Imagine you are driving at night. The headlights of the cars on coming lane will disturb your eye, but you will continue on your way without being too much disturbed. Now imagine you are driving just after the fresh rain. Every single headlight will be like a torture to your eyes. This is nothing but the water filling gaps in the road and shifts its structure from diffuse to specular so you will be affected by two headlights instead of one since the road lost its diffusion, irregularities. See the simple image :



Anisotropy


In 3D anisotropy is used for things such as stretching the reflection in certain directions and changing direction according to the angle viewed. In real life imagine as regular irregularities! Practical uses are generally brushed metal types. A map can also be assigned to define direction.



A radial brushed pot bottom is a good example of Anisotropy. If you search for Radial Brushed Metal and assign one of the images to the Anisotropy channel, you can get a similar result.



Different types of brushing.


Refraction

Refraction is a very extensive subject.

Kırılma çok kapsamlı bir meseledir. We will try to explain some qualities roughly.

We already mentioned what the refraction is. If, for instance a prependicular light refracts less than 90 degree this means light interacted with transparent or translucent object. Most definitive quality of refraction is the Index of Refraction (IOR).



Index Of Refraction (IOR)






In short, IOR indicates how much the light rays will change direction when they enter a medium/interact with an object. According to Snell Law, Mathematical formula is :


n1 sinθ1 = n2 sinθ2


Did I get it? Yesn't. I wasn't fond of Math.


Index value of 1 represents vacuum medium and this means there is no refraction since there is little to no substance in vacuum. As the value increases, the amount of refraction increases. If you want to create realistic materials it is good to know IOR of some general objects. Click here for the list.


Total Internal Reflection (TIR)


This is a special case of Refraction. It has an extensive mathematical explanation.

In simple, if light rays cross a critical angle as they pass through transparent objects with different refractive indices, rays are no longer refracted but returned as reflections.




In this water case, when the angle of incident ray adds with the IOR of the surface and result exceeds the critical angle (which is 90 between flat surface to x axis) it becomes reflection and the total inner angle of incident and returned rays are called TIR.


Some practical examples of this are a swimming pool or an aquarium filled with water.

In the aquarium illusion we have a glass with its own IOR and it is filled with water of its own IOR. When we look at the aquarium from an angle seeing two sides of it, we can clearly see two instances of same fishes. Depending on the angle one of them is real result of refraction rays while the other one is a simple reflection on the surface of glass caused by TIR. Notice that this will not work clearly on round aquariums since there is no edge breaks you will have a very narrow angle to observe just a litte of distorted TIR.


Some sample images:








Dispersion



So far, we have talked about the integral behaviours of light rays, refraction, reflection. Now it is time to break that integrity. Early on we mentioned that the white light or sun light, contains the wavelengths and frequencies of all primary colors. When the light interacts with substances with certain IOR's in a non prependicular angle, refraction strength will differ by the wavelength of light rays, which causes diverging of primary frequencies.


As a basic physical rule, speed ​​is the most important factor of how much direction an object will change when it hits another object.


As a result we see that the red, color of lowest frequency (speed) will change its direction a bit when it enters and leaves that triangular glass prizm while the violet, color of highest frequency (speed) will change its direction much more and the colors between according to their speed, frequency.

(Due to our topic, we are going based on transparent objects, but features such as IOR, TIR, Dispersion are also valid for some non-transparent objects like metals.)


This phenomenon called Dispersion is also called as Abberation or Abbe depending on your render engine.

Some examples of Dispersion are that triangular glass prizm above, rainbow (every drop of rain of partice of dew functions as prizm) and the Chromatic Abberation you see on the edges of the photos taken by cheap DSLR lenses hated by photographers. (Don't play with f-stop, you have a cheesy lens, go get some ED,ELD,ULD Prime lenses)


Period for now. Next we will talk about further deep Translucency/Sub Surface Scattering (SSS) qualities.


Stay safe.








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