What is Signal; sinusoidal signal?

Example 5.14 (Two loudspeakers)

Two speakers $2.0\,\mathrm{m}$ apart play the same $850\,\mathrm{Hz}$ tone in phase ($\lambda = 0.40\,\mathrm{m}$). On the perpendicular bisector $\delta = 0$: loud everywhere. Along the segment joining them, $\delta$ changes by $2\lambda$ per wavelength of displacement, so quiet spots ($\delta = \pm\lambda/2, \pm 3\lambda/2, \dots$) sit every $\lambda/2 = 20\,\mathrm{cm}$ — a standing wave, the subject of the next section. Move one speaker’s wire so it plays in opposite phase, and loud and quiet exchange places.

Example 5.15 (Young’s holes)

Two pinholes $S_1$, $S_2$ a distance $a$ apart, lit by one monochromatic source (so that they emit in phase), send light to a screen at distance $D \gg a$. At a point of the screen at height $x$ from the axis, the path difference is

(for $x \ll D$: $S_{1,2}M = \sqrt{D^2 + (x \mp a/2)^2} \approx D + (x \mp a/2)^2/2D$, and subtract). Bright fringes at $x = n\lambda D/a$: the fringe spacing is

With $a = 0.50\,\mathrm{mm}$, $D = 2.0\,\mathrm{m}$, $\lambda = 633\,\mathrm{nm}$: $i = 2.5\,\mathrm{mm}$ — a ruler measures the wavelength of light. In a medium of index $n$ the path difference to use is the optical path $n \times$ length, since $\lambda = \lambda_0/n$.