Fraunhofer lines are produced by

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?

In the Fraunhofer diffraction experiment, $L$ is the distance between the screen and the obstacle, $b$ is the size of the obstacle, and $\lambda$ is the wavelength of the incident light. The general condition for the applicability of Fraunhofer diffraction is:

$A$ single slit Fraunhofer diffraction pattern is formed with white light. For what wavelength of light does the third secondary maximum in the diffraction pattern coincide with the second secondary maximum in the pattern for red light of wavelength $6500 Å$ (in $Å$)?

In Fraunhofer diffraction,draw the graph of the intensity of light on the screen versus the diffraction angle $\theta$ and write its characteristics.

In a Fresnel diffraction arrangement,the screen is at a distance of $2 \ m$ from a circular aperture. It is found that for light of wavelengths ${\lambda _1}$ and ${\lambda _2},$ the radius of the $4^{th}$ zone for ${\lambda _1}$ coincides with the radius of the $5^{th}$ zone for ${\lambda _2}.$ Then the ratio ${\lambda _1}:{\lambda _2}$ is