# Problem 13-72 Double source interference

An interference pattern using microwaves of wavelength $3.0\; cm$ is set up in a physics laboratory. (Microwaves are part of the electromagnetic spectrum; they travel at a speed of $3.0 \times 10^8\; m/s$ in air.) Two sources of in-phase waves are placed $18 \;cm$ apart and a receiver is located $4.8\; m$ away from the midpoint between the sources.
(a) What is the frequency of the microwaves? Express your answer in megahertz $(MHz)$ and gigahertz $(GHz)$.
(b) As the receiver is moved across the pattern parallel to an imaginary line joining the sources, what is the distance between adjacent maxima, between adjacent minima, and between a maximum and an adjacent minimum?

[Ans. (a) $10000\; MHz; 10\; GHz$  (b) $0.80\; m; 0.80\; m; 0.40\; m$ ]

The correct relationship to solve part (a) is:

(A)  $v= f\lambda$

(B)  $x = ML/d$

(C)  $x = (N - 1)L\lambda/d$