Abstract: The intensity of diffracted light at the geometric centre of the diffraction optical field is always not ze- ro through diffraction by a real obstacle screen such as a disk or a filament. Clearly, this is the best demonstration of the wave nature of light, and so it is more convincing than that of diffraction by an aperture such as a circular ap- erture or a slit. However. the theoretical analysis for the light spot （such as Poisson point） at the obstacle diffrac- tion field center is usually based on the Fresnel diffraction, while for the majority students who are not majored in physics, they are just required to have the knowledge of Fraunhofer diffraction in the course of university physics, thus it causes difficulty to them to understand the related questions. In this paper, the starting point of argument is directly based on the Fraunhofer diffraction which is familiar to the students majored in science and engineering. By use of the Fresnel integral formula or Babinet principle combined with simple analyze method of semi-qualitative and semi-quantitative, we give two easily ways for solving the problem. It is seen that the argument method is helpful for students to understand the relevant questions.

Key words: light diffraction, Huygens-Fresnel theory, Babinet principle, university physics