In YDSE when a point source S₁ of monochromatic light of wavelength λ₁ is used the number of bright fringes obtained on the screen is n₁. If the point source S₁ is replaced by another point source S₂ of monochromatic light of wavelength λ₂(λ₂< λ₁ ), the number of bright fringes obtained on the screen is n₂, then

$n_2 \ge n_1$

$\text{Let width of screen is L}$

$n_1 = \frac{L}{\beta_1} = \frac{L}{\lambda_1 D/d} = \frac{dL}{\lambda_1 D}$

$n_2 = \frac{L}{\beta_2} = \frac{L}{\lambda_2 D/d} = \frac{dL}{\lambda_2 D}$

$\text{Since } \lambda_1 > \lambda_2$

$\Rightarrow n_2 \ge n_1$