Ray optics fails when

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

$A$ slit of size '$a$' is illuminated by a parallel beam of light of wavelength $\lambda$. The angle at which this light is diffracted is approximately . . . . . . .

In a diffraction experiment from a single slit,the angular width of the central maximum does $NOT$ depend upon:

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):

In a single-slit diffraction experiment,the slit is illuminated by light of two wavelengths $\lambda_1$ and $\lambda_2$. It is observed that the $2^{nd}$ order diffraction minimum for $\lambda_1$ coincides with the $3^{rd}$ diffraction minimum for $\lambda_2$. Then: