When Lebesgue of outer measure of an interval is its length?

The Lebesgue outer measure of an interval is equal to its length because the outer measure is defined as the infimum of the sums of the lengths of open intervals that cover the set. For a closed interval ([a, b]), the length is (b - a), and it can be covered exactly by itself, making the infimum equal to this length. Therefore, for intervals, the Lebesgue outer measure coincides precisely with their geometric length.