What is the condition of Nth minima in diffraction?

Draw a labelled graph of the intensity of diffracted light $\left(I\right)$ versus angle$\left(\theta \right)$ in the Fraunhofer diffraction experiment for a single-slit diffraction.

Ray optics is based on the assumption that light travels in a straight line. Diffraction effects (observed when light propagates through small apertures/slits or around small obstacles) disprove this assumption. Yet the ray optics assumption is so commonly used in understanding location and several other properties of images in optical instruments. What is the justification?

A $0.02\mathrm{cm}$ wide slit is illuminated at normal incidence by light of wavelength $6000 \mathrm{\AA }$

$\left(i\right)$ Find the width of the central maximum band on the screen placed $1 \mathrm{m}$ away from the slit.

$\left(ii\right)$ What should be the fringe width if the apparatus is immersed in water whose refractive index is $\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3$}\right. ?$

When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?

In Fraunhoffer diffraction for single slit central maxima is bright on the both side of it, maxima & minima occurs symmetrically.



For Diffraction Maxima: $a\; sin\theta =\left(2n+1\right)\frac{\lambda }{2}$
For Diffraction Minima: $a\; sin\theta =\text{n}\lambda$

Light of wavelength $\lambda$ passes through a single slit of width $W$ and forms a diffraction pattern on a viewing screen. If this light is then replaced by light of wavelength $2\lambda$, the original diffraction pattern is exactly reproduced if the width of the slit is changed to