Diffraction effects are easier to notice in the case of sound waves than in the case of light waves because

$A$ parallel beam of fast-moving electrons is incident normally on a narrow slit. $A$ fluorescent screen is placed at a large distance from the slit. If the speed of the electrons is increased,which of the following statements is correct?

$A$ parallel beam of light of wavelength $500\; nm$ falls on a narrow slit and the resulting diffraction pattern is observed on a screen $1\;m$ away. It is observed that the first minimum is at a distance of $2.5\;mm$ from the centre of the screen. Find the width of the slit. (in $; mm$)

The condition for observing Fraunhofer diffraction from a single slit is that the light wavefront incident on the slit should be

Assertion : When a tiny circular obstacle is placed in the path of light from some distance,a bright spot is seen at the centre of the shadow of the obstacle.
Reason : Destructive interference occurs at the centre of the shadow.

Light of wavelength $\lambda$ is incident on a single slit of width $a$,and the distance between the slit and the screen is $D$. In the diffraction pattern,if the slit width is equal to the width of the central maximum,then $D=$