Wave Optics Questions

In a double slit experiment, when light of wavelength $400\text{nm}$ was used, the angular width of the first minima formed on a screen placed $1\text{m}$ away, was found to be $0.2^{\circ }$. What will be the angular width of the first minima, if the entire experimental apparatus is immersed in water? (${\mu }_{water}=4/3$)

Which colour of the light has the longest wavelength?

In Young's double-slit experiment, if there is no initial phase difference between the light from the two slits, a point on the screen corresponding to the fifth minimum has a path difference:

The angular width of the central maximum in the Fraunhofer diffraction for $\lambda =6000\text{A˚}$ is ${\theta }_0$. When the same slit is illuminated by another monochromatic light, the angular width decreases by $30\%$. The wavelength of this light is: