In the phenomenon of refraction of light for the same angle of incidence

A double slit experiment is immersed in water of refractive index $1.33$. The slit separation is 1 mm, distance between slit and screen is $1.33$ m. The slits are illuminated by light of wavelength 6300 $\text{\AA}$. The fringe width is

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

When a metallic surface is illuminated with a radiation of wavelength '$\lambda$', the stopping potential is 'V'. If the same surface is illuminated with radiation of wavelength $6\lambda$, the stopping potential is $\left(\dfrac{V}{12}\right)$. The threshold wavelength for the surface is