Answer:
Step-by-step explanation:
Distance between fringe or fringe width = xλ / d 
where x is location of screen and d is slit separation 
Given x = 4 m 
λ = 694 nm 
d = .085 x 10⁻³ m 
distance between fringes 
= 4 x 694 x 10⁻⁹ / .085 x 10⁻³
= 4 x 694 x 10⁻⁹ / 85 x 10⁻⁶
= 32.66 x 10⁻³ m
= 32.66 mm . 
3.267 cm 
b )
when submerged in water , wavelength in water becomes as follows 
wavelength in water = wave length / refractive index
= 694 / 1.333 nm 
= 520.63 nm 
new distance between fringes 
3.267 / 1.333
= 2.45 cm .