For a progressive wave, state what the term period means.
State the principle of superposition.
Electromagnetic waves with wavelength $0.040\,\text{m}$ are emitted in phase from two sources X and Y and then travel through a vacuum. Their arrangement is shown in Fig. 4.1. A detector moves along a path parallel to the line XY, and an intensity pattern of maxima and minima is observed. Distance XZ is $1.380\,\text{m}$ and distance YZ is $1.240\,\text{m}$. State the name of the region of the electromagnetic spectrum that includes the waves from X and Y.
Calculate the period, in $\text{ps}$, for the waves.
Show that the path difference at point $Z$ between the waves from $X$ and $Y$ is $3.5\lambda$, where $\lambda$ is their wavelength.
Calculate the phase difference between the two waves at point $Z$.
At point $Z$, the waves from $X$ alone have the same amplitude as those from $Y$ alone. State the intensity of the waves at point $Z$.
The frequencies of the waves from $X$ and $Y$ are both reduced to the same lower value. The waves remain in the same region of the electromagnetic spectrum. Describe the effect of this change on the pattern of intensity maxima and minima along the path of the detector.