When a photon enters glass from air, which one of the following quantity does not change?

In Young's double slit experiment fifth dark fringe is formed opposite to one of the slit. IID is the distance between the slits and the screen and $d$ is the separation between the slits, then the wavelength of light used is

The phenomenon of interference is based on

Light of wavelength ' $\lambda$ ' is incident on a single slit of width ' $a$ ' and the distance between slit and screen is ' $D$ '. In diffraction pattern, if slit width is equal to the width of the central maximum then ' $D$ ' is equal to