If the 8th bright band of wavelength $λ_1$ coincides with 9th bright brand of wavelength $λ_2$ in a Young's double slits experiment then the possible wavelength of two lights are:

$λ_1=450 nm, λ_2=400 nm$

The correct answer is Option (1) → $λ_1=450 nm, λ_2=400 nm$

In young's double slit experiment,

$mλ_1=nλ_2$

$⇒8λ_1=9λ_2$

$⇒\frac{λ_1}{λ_2}=\frac{9}{8}$

Now,

$\frac{λ_1}{λ_2}=\frac{450}{400}=\frac{9}{8}$