Answer:
The required wavelength is 1.19 μm
Step-by-step explanation:
In the double-slit study, the formula below determines the position of light fringes
on-screen.

where;
m = fringe order
d = slit separation
λ = wavelength
D = distance between screen to the source
For the first bright fringe, m = 1, and we make (d) the subject, we have:


replacing the value from the given question, we get:

In the double-slit study, the formula which illustrates the position of dark fringes
on-screen can be illustrated as:

The value of m in the dark fringe first order = 0
∴


making λ the subject of the formula, we have:

