For this, take a chart paper, a torch and a plane mirror. Paste a black paper on the front of the torch and make a small hole at the centre of the black cover. This will give you a thin beam of light. Keep the chart paper on a table in a way that some portion of the chart paper is outside the edge of the table. Keep the mirror vertically on the chart paper. Direct the beam of light on the mirror so that the incident ray is along the surface of the chart paper. Mark the incident ray, the normal and the reflected ray. After that, fold the portion of the chart paper which is jutting out of the table. Repeat the above step and observe the incident ray and reflected ray. It is observed that the reflected ray cannot be seen on the folded portion of the chart paper. The folded portion of the chart paper is in a different plane than the remaining part of the chart paper. This proves that the incident ray, the normal and the reflected ray; all lie in the same plane.