Knowledge about shadows

1. What is a shadow? Light travels in a straight line, and the object in front of the light source blocks the light, and a dark area called a shadow is formed where the light cannot shine. The reason why we see this area is dark is because the light can't reach the back of the blocked object and can't be reflected into our eyes.

An object can cast shadows of different shapes. When the object is close to the light source, the shadow is larger because the object blocks a lot of light. As the object moves away from the light source, the shadow becomes smaller because the light blocked by the object decreases. If you rotate an object, you will get different shapes of shadows.

The edge of the shadow may be clear or blurred. When an object is close to a place where a shadow is cast, such as a wall, the edge is clearly visible. When the object is far away from the place where the shadow is cast, the edge will become blurred. Sometimes the middle part of the shadow is darker than the edge, the middle part is called umbra, and the brighter area is called penumbra. This shadow will appear when the light source is large. In order to make all parts of the shadow equally dark, the light source must be small.

The shadow outside depends on the weather. In sunny, cloudless and sunny weather in Wan Li, the shadows are clear and dark. When the weather is cloudy and gloomy, it is difficult to see the shadow, because the sunlight is scattered by the tiny particles in the cloud and emitted in many directions, which means that the shadow is brighter than usual, so it is not easy to see.

2. The application of shadows by human beings has been very useful to human beings since ancient times. The movement of shadow has its fixed pattern, so we can predict the size and position of shadow. When the earth rotates around its axis, everything on the earth's surface is illuminated by the sun. Mountains, trees and buildings all have shadows, but the position of shadows changes constantly throughout the day. As the position of the sun changes, the shadow moves from one side of the object to the other. In the early morning and evening, the sun in the sky is very low, and the shadows are very long at these times. At noon, the sun is high in the sky and the shadow is very short. The ancients found that the passage of time could be marked with a note, so they invented a sundial to let people know the local time.

3. The color of the shadow The color of the shadow depends on the light reflected by the shadow projection, and then the light blocked by the object is deducted. For example, a white wall reflects white light (in fact, white light contains various colors), so it looks white. When a part of the white light is blocked by an object, the light projected by the object on the wall becomes less, so the shadow part looks darker than the original wall.

If the object selectively blocks the light, the shadow will show different color changes instead of simple brightness changes. There are two possibilities as to how to selectively block the light. First of all, the object itself is selectively transparent (such as colored glass), so it will change the color of the shadow. Secondly, there are more than one light source and different colors. The object only covers part of the light source. For example, if two light sources, red light and green light, are installed in different positions in the environment, the same object will cast shadows of two different colors, red and green, because it is illuminated by both red light and green light, it will appear yellow.