Everyone would have travelled on aeroplanes but never would have noticed why they have round windows & not square windows.
Well, its for the safety that the windows are round & not square
When Square windows were used in the year 1953, 2 planes fell apart in air, thus killing 56 people. Windows were the reason behind this crash!
Because if there be a corner, there will be a weak spot. Windows with 4 corners have 4 weak spots and they can crash easily under air pressure. Stress concentration on corners is more than the round surfaces.
When the window is curved, the stress that cracks the window corner is distributed and there is reduction in the likelihood of windows breaking. Watch the video below to understand better.