AL phy wave 2001paper 1

(b) wavelength 入 = 3x10^8/60x 10^6 m = 5 m

where 3x10^8 is the speed of electromagentic wave in airFor destructive interference to occur, path difference = m入 + 入/2
where m is an integer (order of interference)

Hence, QB - PB = m入 + 入/2 -------------------- (1)
and QA - PA = (m+1)入 + 入/2 ------------------- (2)
(1) - (2): (QA - QB) + (PB - PA) = 入
i.e. (AB) + (AB) = 入
2(AB) = 入
hence, AB = 入/2 = 5/2 m = 2.5 m

AB is the least separation because their order of interference differs by unity.

(c) The lines are just nodal lines of interference pattern produce by two coherent point sources.

You may refer to the following web-page (press the "pause" button to frozen the motion)

http://www.walter-fendt.de/ph14e/interference.htm