This experiment raises at least three questions: why is the image of a small hole upside down. What factors are related to the size of the image. What factors are related to the clarity of the image.

In order to illustrate these problems, we regard the flame of a candle as composed of many small luminous points, each of which emits light in all directions. There will always be a small beam of light, straight through the small hole, forming a small spot on the white paper. Each luminous point on the candle flame will form a corresponding spot on the white paper, and all spots on the white paper form an image of the candle flame.

It can be seen from the figure that the light from the upper part of the candle flame passes through the small hole along a straight line and shines on the lower part of the white paper; The light from the lower part of the candle flame shines on the upper part of the white paper through a small hole, so an inverted image is formed on the white paper. This just shows that light travels in a straight line.