In biprism experiment, $$6^{\text {th }}$$ bright band with wavelength '$$\lambda_1$$' coincides with $$7^{\text {th }}$$ dark band with wavelength '$$\lambda_2$$' then the ratio $$\lambda_1: \lambda_2$$ is (other setting remains the same)

In Young's experiment with a monochromatic source and two slits, one of the slits is covered with black opaque paper, the fringes will

In the interference experiment using a biprism, the distance of the slits from the screen is increased by $$25 \%$$ and the separation between the slits is halved. If '$$W$$' represents the original fringewidth, the new fringewidth is

In biprims experiment, the $$4^{\text {th }}$$ dark band is formed opposite to one of the slits. The wavelength of light used is $$(\mathrm{d}=$$ distance between the slits, $$\mathrm{D}=$$ distance between scource and the screen)