Why can we see shadows in small hole imaging experiments?

1. Ancient astronomical calendar

The linear propagation properties of light have been widely used in ancient Chinese astronomical calendars. Our ancestors made watches and sundials to measure the length and direction of the sun's shadow to determine the time, winter solstice, and summer solstice. They also installed sight tubes on astronomical instruments to observe celestial phenomena and measure the positions of stars.

In addition, our country has long used this property of light to invent shadow puppetry. In the early Han Dynasty, Qi Shaowen performed a performance with paper-cut people and objects behind a white curtain, and illuminated it with light. The images of the people and objects were reflected on the white curtain, and people outside the curtain could see the performance of the images. Shadow puppetry became very popular in the Song Dynasty and later spread to the West, causing a sensation.

2. Cameras and video cameras

Some cameras and video cameras use the principle of small hole imaging - the lens is a small hole (most of them are equipped with convex lenses to ensure the imaging distance of light), and the scenery Entering the darkroom through a small hole, the image is left on the film by some special chemicals (such as silver halide, etc.) (digital cameras, video cameras, etc. store the image in the memory card through some photosensitive elements).

3. The wave theory of light

It plays a key role in the wave theory of light, such as Young's interference experiment, double slit experiment and other light interference experiments. Small hole imaging The principles and methods of interference experiments are still very useful in modern physics.

Small hole imaging demonstration:

Use a sharpened and thin pencil to poke a small hole circle in the center of a piece of cardboard.

The diameter of the small hole is about three millimeters. Try to stand it upright on the table, and then draw the curtains to dim the light in the room.

Light a candle and place it near the hole. Take a piece of white paper and place it on the other side of the hole. In this way, you will see an inverted candle flame on the white paper. We call it the image of a candle. Move the white paper back and forth and see how the image of the candle flame changes. When the white paper is closer to the candle, the image is small and bright; when the white paper slowly moves away from the candle, the image gradually becomes larger and the brightness becomes darker.

Change the size of the hole, and let’s observe the changes in the image of the candle.

You can punch several holes of different sizes and shapes on the cardboard, with a few centimeters between the holes. At this time, several inverted images corresponding to the small holes appeared on the white paper. They are all the same size, but have different degrees of clarity. The larger the hole, the clearer the image. As long as the hole is small enough, its shape, whether square, round, or oblate, has no effect on the clarity or shape of the image.