Ray optics fails when

The penetration of light into the region of geometrical shadow is called:

$A$ plane wavefront of wavelength $6 \times 10^{-7} \,m$ is incident on a slit of width $0.4 \,mm$. $A$ convex lens of focal length $0.8 \,m$ is placed behind the slit to form a diffraction pattern on a screen. What is the linear width of the second secondary maximum in $mm$?

$A$ slit of width $12 \times 10^{-5} \ cm$ is illuminated by monochromatic light of wavelength $6000 \ \mathring A$. Find the half angular width of the central bright maximum in the Fraunhofer diffraction pattern in degrees $(^o)$.

Light of wavelength $600 \,nm$ is incident normally on a slit of width $0.2 \,mm$. The angular width of the central maxima in the diffraction pattern is (measured from minimum to minimum):

Consider diffraction of light through a rectangular slit which is twice as wide as it is high. Which of the following statements is true?