For what distance is ray optics a good approximation when the aperture is $ 4 \,mm $ and the wavelength of light is $ 400 \,nm $ (in $\,m$)?

$A$ converging lens of a telescope with a diameter of $5 \text{ cm}$ has a focal length of $25 \text{ cm}$. In the focal plane of the lens, the distance between the centres of the Fraunhofer diffraction pattern is (wavelength of light used = $5000 \text{ Å}$, Rayleigh's criterion is satisfied).

In a Fraunhofer diffraction pattern,the slit width is $0.2 \ mm$ and the screen is at $2 \ m$ away from the lens. If the distance between the first minimum on either side of the central maximum is $1 \ cm$,the wavelength of light used is: (in $Å$)

Find the half-angular width of the central maximum in the Fraunhofer diffraction pattern produced by a slit of width $12 \times 10^{-5} \text{ cm}$,when illuminated by monochromatic light of wavelength $6000 \, \mathring{A}$. (Answer in degrees)

What will be the angle of diffraction for the first minimum due to Fraunhofer diffraction with a source of light of wavelength $550 \,nm$ and a slit of width $0.55 \,mm$ (in $,rad$)?

Find the correct condition$(s)$ for Fraunhofer diffraction due to a single slit.