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Showing posts from 2019


New word: Borissing

1. A behavior who started something, but succeeded to running away without taking responsibility.
Usage example: a school teacher told a student, "Please don't borissing. You are already a 5th grade."

2. Referring an social harassment. Some people coincidentally has a similar name. There are some incidents, such child was picked in a class. The association "We did nothing wrong" officially stated, this is a discrimination, please stop using the word. But another association, "Yes, but this is the consequence" stated recently, we should solve the problem from the source, otherwise we could not completely solve this problem.

-- in the Devil's dictionary draft

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. 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 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 length difference \(\Delta x_r\) is slightly differ from the \(\Delta x\). The real path length difference \(\Delta x_r\) is
\[ d \sin (\theta - \alpha) = \Delta x_r,\]
where the distance b…