$A$ 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 $Å$)?

For an aperture of $5 \times 10^{-3} \ m$ and a monochromatic light of wavelength $\lambda$, the distance for which ray optics becomes a good approximation is $50 \ m$, then $\lambda=$ (in $\text{Å}$)

The cause of Fraunhofer lines is

The direction of the first secondary maximum in the Fraunhofer diffraction pattern at a single slit is given by ($a$ is the width of the slit):

Consider the following statements $A$ and $B$ and identify the correct answer:
$A$. Fresnel's diffraction pattern occurs when the source of light or the screen on which the diffraction pattern is seen,or when both are at a finite distance from the aperture.
$B$. Diffracted light can be used to estimate the helical structure of nucleic acids.

The condition for observing Fraunhofer diffraction from a single slit is that the wave front incident on the slit should be ............