What will be the angular width of the central maxima in Fraunhofer diffraction when light of wavelength $6000 \,\mathring{A}$ is used and the slit width is $12 \times 10^{-5} \, \text{cm}$? (in $rad$)

For Fraunhofer diffraction to occur:

Estimate the distance for which ray optics is a good approximation for an aperture of $4 \; mm$ and wavelength $400 \; nm$.

In a Fraunhofer diffraction pattern,the slit width is $0.5 \ mm$ and the screen is at $2 \ m$ away from the lens. If the wavelength of the light used is $5500 \ Å$,then the distance between the first minima on either side of the central maximum is (assume $\theta$ is small and measured in radians). (in $mm$)

Two towers on top of two hills are $40 \, km$ apart. The line joining them passes $50 \, m$ above a hill halfway between the towers. What is the longest wavelength of radio waves (in $cm$),which can be sent between the towers without appreciable diffraction effects?

If the slit width is $2 \, mm$ and the wavelength of light used is $4000 \, Å$, then the Fresnel distance is nearly: