Q.

In a double slit experiment instead of taking slits of equal widths, one slit is made twice as wide as the other. Then, in the interference pattern                [2000]

1 the intensities of both the maxima and the minima increase  
2 the intensity of the maxima increases and the minima has zero intensity  
3 the intensity of the maxima decreases and that of the minima increases  
4 the intensity of the maxima decreases and the minima has zero intensity  

Ans.

(1)

When slits are of equal width,

      Imax∝(a+a)2=(4a2)

       Imin∝(a-a)2=(0)

When one slit's width is twice that of the other,

       I1I2=W1W2=a2b2⇒W2W=a2b2⇒b=2a

∴  Imax∝(a+2a)2=(5.8 a2)

       Imin∝(2a-a)2=(0.17 a2)

Clearly, the intensities of both the maxima and minima increase.