## Youngs double slit related problems

Gday, im having some trouble with these questions becuase i was away for the two weeks when we did them any help or explanation would be greatly appreciated.

1) A transmitter drivest wo radio antenna A and B placed 2$$\lambda$$ appart. Each atenna emit radio waves, in phase, uniformly in all directions. Angle $$\theta$$ defines the direction to a distant point P. Find the value of $$\theta$$ if there is

a) Constructive interference at P
b) Destructive interference at P

2) A light source emits light of two distinct wavelengths. One of these is 640 nm.
a) If the third dark band from the centre line of this wavelength conicides with the fourth dark band of the other wavelength when both wavelengths pass through the same doubleslits what is the second wavelength?

thank you, Alex ( i really don't know where to start)
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