When you induce stress on an object and see it through your Polaroid sunglass, then you witness these amazing rainbow patterns.
This property of a material where the changes in optical properties of a material is used to determine its stress distribution is known as Photoelasticity.
The simplest way to understand stress distribution in a material is by using a sponge and some wooden planks.
Observe how the lines on the sponge change when one applies the load:
Uniformly Distributed Load
When a concentrated load is applied, the lines closer to the loading point become radially distorted but the effect of this distortion dies out as moves away. *
If those lines made sense to you, then the lines that you see through your polarized sunglasses are no different.
In addition, the patterns that you observe are directly proportional to load that you apply. You vary the load, you vary the pattern observed.
Why are airplane windows round?
How does knowing the stress concentration help you at all ? When you are an Engineer, knowing the stress concentration tells you the critical stress points in a structure ( or points of probable easiest failure )
Stress concentration in Square v/s Oval windows
As this Real Engineering video goes on to explain when square windows are used in an aircraft, there is a greater accumulation of stress in the edges than the oval windows.
This increased stress, lead to cracks forming near the sharp edges of the window and causing major havoc, which is why all modern aircraft windows are round.