According to the equal optical path of the lens, the optical path of light from the upper part of the slit to the vertical line (dotted line in the figure) to point P is equal, and the optical path difference of light 1, 2, 3 is only on the section from the slit to the dotted line. As can be seen from the figure, the optical path difference between light 1 and 3 is 2λ, so it can be judged that light 2 is between light 1 and 3, which is different from light 65438+.

According to the relationship between optical path difference and phase difference: φ = 2π δ/λ.

Therefore, the phase difference between light 1 and 2 at point p is 2π.

According to the half-wave band analysis method, the slit can be divided into four half-wave bands, and the light waves emitted by adjacent half-wave bands meet at point P to cancel each other out, so the light waves meet at point P in the figure as dark lines with two levels of dark lines.