Given that
d=0.5mm=5×10⁻³m
D=1m
5th bright form 2nd dark
y5th bright=(7/2 ×λD/d)=4.13 mm = fringe width
β=(λD/d) ⇒7β/2 ×λD/d=4.13×10⁻³
2×λD/2d=4.13×10⁻³
λD/d=4.13×10⁻³
λ×1/(5×10⁻³)=4.13×10⁻³
λ=4.13×5×10⁻³×10⁻³
λ=20.65×10⁻⁶m
λ=20650×10⁻¹⁰m
λ=20650Å
Thus, wavelength of light used=λ=5900Å