Duduli

Answer:- A zone plate is a specially constructed screen such that light is obstructed  from every alternate zone.It can be designed so as to cut off light due to the even numbered zones or that due to the odd numbered zones.

Answer:- If two or more waves are in same phase or having a constant phase difference over a long distance and time then they are said to be coherent waves.

Answer:- “Wavefront of a wave is defined as the locus of all the particles of the medium which vibrate in same phase and were disturbed at the same instant of time.”