Who was the first to study the phenomenon of interference of light?

In a $YDSE$ with two identical slits,when the upper slit is covered with a thin,perfectly transparent sheet of mica,the intensity at the centre of the screen reduces to $75\%$ of the initial value. The second minimum is observed to be above this point and the third maximum below it. Which of the following cannot be a possible value of the phase difference caused by the mica sheet?

In Young's double slit experiment,the central bright fringe can be identified

In an interference pattern,if the ratio of slit widths is $1:9$,find the ratio of maximum to minimum intensity.

In Young's double-slit experiment,the two slits are $2 \, mm$ apart and are illuminated by a mixture of two wavelengths $\lambda_1 = 12000 \, \mathring{A}$ and $\lambda_2 = 10000 \, \mathring{A}$. At what minimum distance from the common central bright fringe on the screen will a bright fringe from one interference pattern coincide with a bright fringe from the other? The distance between the slits and the screen is $2 \, m$.

If the path difference of the point on the screen from two coherent sources is constant, what is the shape of the trajectory of the point on the screen?