Suppose there exist infinitely many equally spaced parallel lines, and we were to randomly toss a needle whose length is less than or equal tothe distance between adjacent lines. What is the probability that the needle will lie across a line upon landing? To solve this problem, let be the length of the needle and be the distance between two adjacent lines. Then, let be the acute angle the needle makes with the horizontal, and let be the distance from the center of the needle to the nearest line. The needle lies across the nearest line if and only if. We see this condition from the right triangle formed by the needle, the nearest line, and the line of length when the needle lies across the nearest line. Now, we assume that the values of are randomly determined when they land, where, since, and. The sample space for is thus a rectangle of side lengths and. The probability of the event that the needle lies across the nearest line is the fraction of the sample space that intersects with. Since, the area of this intersection is given by Now, the area of the sample space is Hence, the probability of the event is
Bending the needle
The interesting thing about the formula is that it stays the same even when you bend the needle in any way you want, making it a "noodle"—a rigid plane curve. We drop the assumption that the length of the noodle is no more than the distance between the parallel lines. The probability distribution of the number of crossings depends on the shape of the noodle, but the expected number of crossings does not; it depends only on the length L of the noodle and the distance D between the parallel lines. This fact may be proved as follows. First suppose the noodle is piecewise linear, i.e. consists of n straight pieces. Let Xi be the number of times the ith piece crosses one of the parallel lines. These random variables are not independent, but the expectations are still additive due to the linearity of expectation: Regarding a curved noodle as the limit of a sequence of piecewise linear noodles, we conclude that the expected number of crossings per toss is proportional to the length; it is some constant times the length L. Then the problem is to find the constant. In case the noodle is a circle of diameter equal to the distance D between the parallel lines, then L = πD and the number of crossings is exactly 2, with probability 1. So when L = πD then the expected number of crossings is 2. Therefore, the expected number of crossings must be 2L/. There is one more surprising consequence. In case the noodle is any closed curve of constant width D the number of crossings is also exactly 2. This implies Barbier's theorem asserting that the perimeter is the same as that of a circle.
