The interference and diffraction envelopes of a double slit are shown separately but to the same scale in the figure. For this arrangement, the number of fringes in the second diffraction maximum is
A) 3
B) 4
C) 5
D) 7
E) 0

Question

The interference and diffraction envelopes of a double slit are shown separately but to the same scale in the figure. For this arrangement, the number of fringes in the second diffraction maximum is
A) 3 
B) 4 
C) 5 
D) 7 
E) 0

Quizplus · Accepted Answer

In the second diffraction maximum, the central maximum of the diffraction envelope coincides with the first minimum of the interference envelope. Thus, the distance from the center of the pattern to the first minimum is equal to the distance from the center of the pattern to the second maximum. Counting the fringes from the first maximum, we see that there are 4 fringes between the first minimum and the central maximum, and 3 fringes between the central maximum and the second maximum. Therefore, the total number of fringes in the second diffraction maximum is 4+3=7, which corresponds to answer choice D.

The Interference and Diffraction Envelopes of a Double Slit Are