9702/Oct Nov/12/2016/Q29

A microwave transmitter is placed at a fixed distance from a flat reflecting surface, as shown.

A microwave detector is moved steadily in a straight line from X to Y. A series of maxima and

minima of intensity is obtained. The distance between adjacent maxima is 1.5 cm.
What is the frequency of the microwave radiation?

A 1.0 × 10⁸Hz

B 2.0 × 10⁸Hz

C 1.0 × 10¹⁰Hz

D 2.0 × 10¹⁰Hz

Solution:
Answer: C

distance between adjacent maxima = λ /2

f = v/λ 
f = (3.0 x 10⁸ ms^-1) / (2 x 1.5 x 10^-2 m)

  = 1.0 x 10¹⁰ Hz

*To answer this question successfully, candidates needed to realise that the distance between intensity maxima in a stationary wave pattern is half the wavelength of the wave. Many candidates chose D because they thought that the measured distance was equal to the wavelength.

Reference: PYQ - Oct/Nov 2016 Paper 12 Q29