At a point on the screen directly in front of one of the slits, find the missing wavelengths.

Explanation

According to theory of interference, positon y of a point on the screen is given by
`y = (D)/(d) (Delta x)`
As for missing wavelength, intensity will be minimum `(= 0)`, so
`Delta x = (2n - 1) (lambda)/(2)`
`:. Y = D ((2n - 1) lambda)/(2d)`
However, here `d = b` and `y = (b//2).` Hence,
`lambda = (b_(2))/((2n -1 )D` with `n = 1,2,3,...`
i.e. wavelength `(b^(2)//D), (b^(2)//3D), (b^(2)//5D)`, etc., will be absent (or missing) at point P.