What is the AMPLITUDE AND INTENSITY of multiple slit diffraction????

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Answer 1

When working with light diffraction problems, one must always be aware of the wave properties of light, which involves a phase. Phase itself cannot actually measured directly, but only inferred as an relative attribute. But intensity is something that we do measure directly, it's simply the total energy flux. Any photodetector can measure intensity directly. The amplitude of an electromagnetic wave is the strength of the electric field vector, which has a direction, while the intensity is proportional to the square of the ampliltude. The most common graphical representation of multiple diffraction patterns is the intensity function because it's directly observed in experiments, but occasionally the amplitude function is given, which looks different and not directly measured by apparatus.

The 2nd link mathematically explains the distinction between wave amplitude and intensity.