(a) Define a wavefront. Using Huygen's principle, verify the laws of reflection at a plane surface .
(b) In a single slit diffraction experiment the wildth of the slit is made double the orignal width.
How does this affect the size and intensity of the central diffraction band ? Explain.
(c) When a tiny circular obstacle in placed in the path of light from a distance source, a bright spot is seen at the centre of the obstacle . Explain why.

Explanation

(a) Wavefront: A wavefront is define as the continous locus of all such particle of the medium which are vibrating in same phase at any instant.
There are different type of wavefront.
(i) Spherical wavefront (ii) Plane wavefront (iii) Cylinderical wavefront.
`BC = BC`
`/_ BAC = /_BDC = 90^(@)`
`AC = BD`
`DeltaABC = DetlaADC `
`i = r`
(b) In diffraction, `d sin theta = lambda`
`dtheta = lambda`
`theta = lambda //d`
Linear width of central maximun : If D is the distance of the screen from the single slit then linear width of central maximum will be.
`beta_(0) = D xx 2theta = (2Dlambda)/(d)`
If we increase the size with 2d the `beta_(0) = (2Dlambda)/(2d)`
`2beta = beta_(0) " "beta = (beta_(0))/(2)`
Intensity of central maximum becomes 4 times.
(c) When tiny circular obstacle is placed in the path of light from a distance source a bright spot is seen at the centre.
light when passes from broder of circular obstacle is diffracts towards centre of the obstacle in centre part all diffracted wavefront meet. That is why centre becomes bright.