Why does the intensity vary when the detector is moved away from the slits?

[guest948]

Homework Statement

A student directs a beam of microwaves ... (Please refer to the attachment.)

Relevant Equations

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I guess C: constructive interference pattern along the central line.

Am I correct?

 

[DaveE]

C: constructive interference pattern along the central line.

If the two paths (through each slit) add constructively, as you guessed, then why would the intensity measured increase and decrease with distance as shown in drawing C?

What do you know about the two path lengths from the source to the detector? What does that mean for the phase relationship between them?

 

[guest948]

then why would the intensity measured increase and decrease with distance as shown in drawing C?

The path lengths are the same, so they are in phase and they add constructively along PQ.
So the answer is C (or A)?

 

[DaveE]

The path lengths are the same, so they are in phase and they add constructively along PQ.
So the answer is C (or A)?

The detector can't respond fast enough to measure the instantaneous energy in the radio wave, it only measures the time averaged intensity of the radiation. So why then would the intensity increase and decrease as the detector is moved away from the slits?