Answer:
When r1 – r2 equal the wavelength of the monochromatic light
Explanation:
This is because to observe a bright fringe a constructive interference pattern of path difference, r1 – r2 = mλ is observed, where m is an integer and λ = wavelength of monochromatic light.
For bright fringes and thus constructive interference, r1 – r2 must be integral multiples of the wavelength, λ. When r1 – r2 , we have the central bright fringe and thus m = 0.
The first order bright fringe which is adjacent to the central bright fringe is obtained when m = 1,
So r1 – r2 = 1 × λ
r1 – r2 = λ
Thus, the path difference required to produce the first order bright fringe that is adjacent to the central bright fringe is when r1 – r2 = λ, the wavelength of the monochromatic light.
