To solve this problem it is necessary to apply the concepts related to the principle of overlap and constructive interference.
By definition we know that
[tex]dsin\theta = n\lambda[/tex]
Where,
d = Distance between slits
n = Number of fringes (or number of repetition of the spectrum)
[tex]\lambda[/tex] = Wavelength
Our values are given as
[tex]d = 6.95*10^{-6}m[/tex]
[tex]\theta = 90\° \rightarrow[/tex] The angle is 90 degrees because the angle is the furthest angle the light can rotate from out of the slits.
[tex]\lambda = 625*10^{-9}m[/tex]
Replacing we have that
dsin\theta = n\lambda
[tex](2.95*10^{-6})sin(90) = n(625*10^{-9})[/tex]
n = \frac{(2.95*10^{-6})}{(625*10^{-9})}
n = 4.72
Therefore 4 complete bright fringes are formed and the number of bright fringes without including the central bright fringe is 3. The correct answer is B.
