Step-by-step explanation:
It is given that,
Slit width,

A diffraction pattern is formed on a flat screen located 4.0 m away, L = 4 m
The distance between the middle of the central bright fringe and the first dark fringe is 4.2 mm, y = 4.2 mm = 0.0042 m
Let
is the wavelength of the light.
Using condition of diffraction as,

..............(1)
Also,




So, the wavelength of the light is 596 nm. Hence, this is the required solution.