Q: In young’s double slit experiment, the wavelength of red light is 7800 A° and that of blue light is 5200 A° . The value of n for which nth bright bond due to red light coincides with (n + 1)th bright bond due to blue light, is

Sol: $\large \frac{n_r \lambda_r D}{d} = \frac{n_b \lambda_b D}{d} $

$\large \frac{n_r}{n_b} = \frac{\lambda_b }{\lambda_r} $

$\large \frac{n_r}{n_b} = \frac{5200 }{7800} = \frac{2}{3} $

Therefore 2nd of red coincides with 3rd of blue.