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:

$A$ 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 $= 320 \,m/s$):

The distance for which ray optics becomes a good approximation for an aperture of $0.3 \ cm$ and a light of wavelength $6000 \ Å$ is (in $m$)

Explain that for a greater distance,the spreading due to diffraction dominates over that due to ray optics. $OR$ Explain the importance of Fresnel distance.

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

The Fraunhofer spectrum is a