Two identical sources in phase, each with amplitude $A,$ produce the interference pattern shown in Figure $1.$ Below is a list of labels. Add the labels in their correct places.

lines of destructive interference $($nodal lines$)$

amplitude here is $2A$

crests

areas of constructive interference

amplitude here is $-2A$

troughs

$!$

node

Figure $1$

Two sources are $10\mathrm{.}0cm$ apart. An interference pattern is observed at a distance of $25\mathrm{.}0cm$ along the right bisector of the line segment joining the sources. The third nodal line is $33\mathrm{.}6cm$ from the right bisector at this distance

Waves of wavelength $8\mathrm{.}12cm$ are produced by two sources vibrating in phase with the same amplitude. What is the path length difference for a point on the fourth nodal line from the centre of the interference pattern? $$