Consider interference between two sources of intensity I and 4I. Obtain intensity at a point where phase difference is <pi>/2.

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My teacher gave this formula  : I = a₁² +a₂² + 2a₁a₂cos@ (i have no idea about derivation)

According to this, answer comes to 5I..but this is destructive interference!!

What's the correct formula to be used?

If possible, please tell how to derive it also..

Rates assured!

Thanks!!:-)

Please help!

formula to be used

so this give I=3I

   so formula modifies to    

 ur answer is correct but its not fully destructive interference where phase diff =pie

nor fully constructive

THE CASE HERE CORRESPONDS TO AVG VALUE OF I MAX AND I MIN

 I MEAN THE INTENSITY INCREASES BECAUSE ITS NOT DESTRUCTIVE INTERFERENCE!!!!!

Guys which is the correct formula ?The one given by eshaan or by divinexsoul?

And how is this formula derived?

Ok,could you tell me how its derived?

Thanks:-)

go  to  http://www.goiit.com/chapters/tutorial/physics/superposition-3.htm

Thanks!:-)