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?

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

Consider the following statements $A$ and $B$ and identify the correct answer:
$A$. Fresnel's diffraction pattern occurs when the source of light or the screen on which the diffraction pattern is seen, or when both are at a finite distance from the aperture.
$B$. Diffracted light can be used to estimate the helical structure of nucleic acids.

Fresnel diffraction is produced due to light rays falling on a small obstacle. The intensity of light at a point on a screen beyond an obstacle depends on

For what distance is ray optics a good approximation when the aperture is $3\; mm$ wide and the wavelength is $500\; nm$?

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