Young's double slit equation: Many textbooks draw parallel rays, but how could parallel rays intersect?
Young's double slit equation is well known result that shows the light has a wave property. https://www.khanacademy.org/science/physics/light-waves/interference-of-light-waves/v/youngs-double-slit-part-2 https://en.wikipedia.org/wiki/Young%27s_interference_experiment A typical illustration of Young's interference experiment is shown in Figure 1. This figure confuses novice learners since at the point C, two parallel rays intersect. But how could possibly parallel rays intersect? In Euclidean geometry, parallel two lines will never intersect. Figure 1: Typical explanation figure: Two parallel rays and path length difference \[\Delta x\] Figure 2: Precise figure: Two rays intersect at point C to cause interference. Figure 2 shows more precise figure of Young's interference experiment. At the point C, two rays meet and cause an interference depends on the path difference from the holes H1 and H2. When you zoom in to the two holes part, you see the precise path