Question

Why diffraction occurs at wavelength comparable to the size of object? Why not we link diffraction with amplitude of wave?

Answer 1

Diffraction can't be linked with amplitude. Amplitude is a factor that determines only the brightness of the light or magnitude (number of light rays).

We came to a conclusion that, 'diffraction occurs at wavelength comparable to the size of object' from experimental data. No scientist have actually found the reason for this phenomenon.

It is experimentally proven and could be explained using complex mathematical data (with several assumptions). However this would be out of scope for discussion here or in your present class. If you go for further advanced studies in wave optics, you will study about this.