For what distance is ray optics a good approximation when the aperture is $4 \, mm$ wide and the wavelength is $500 \, nm$ (in $, m$)?

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

$A$ student is jogging on a straight path with a speed of $5.4 \,km/h$. Perpendicular to the path is a pipe with its opening $8 \,m$ from the road (see figure). The diameter of the pipe is $0.45 \,m$. At the other end of the pipe is a speaker emitting sound of $1280 \,Hz$ towards the opening of the pipe. As the student passes in front of the pipe,she hears the speaker sound for $T$ seconds. $T$ is in the range (Take speed of sound $= 320 \,m/s$):

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

What do the Fraunhofer lines in the solar spectrum indicate?

Fraunhofer lines are produced by