What is Coherent sources, path difference? — Physics

Definition 22.10 High School Physics · Chapter 22 — Light as a Wave: Diffraction and Interference

Two wave sources are coherent when they have the same frequency and vibrate in step — in practice, when they are two copies of one wave, obtained by splitting it. For a point $M$ reached by both, the path difference is $\delta = S_2M - S_1M$ .

Young’s slits: the paths to M differ by = S_2M - S_1M; bright fringes (gray ticks) sit where = k. — Young’s slits: the paths to $M$ differ by $\delta = S_2M - S_1M$ ; bright fringes (gray ticks) sit where $\delta = k\lambda$ .

Examples

Example 22.12 (Young’s two-slit experiment)

Pierce two fine slits $S_1$ , $S_2$ , a fraction of a millimetre apart, in an opaque plate, and light both with one laser: two coherent sources. On a screen a couple of metres away the overlap is striped: at the center $\delta = 0$ , bright; climbing the screen, $\delta$ grows through $\lambda/2$ (dark), $\lambda$ (bright) … darkness where light added to light cancels.

Example 22.18 (Soap films and oil slicks)

Light striking a soap film reflects partly off its front face, partly off its back: two coherent copies whose path difference is set by the thickness and the viewing angle. Each thickness returns some wavelengths reinforced, others canceled: the film reflects a color that slides as it drains and thins. An oil slick on wet asphalt plays the same trick.

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Examples

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