As the picture shows, how do shadows, hand shadows, solar eclipses and lunar eclipses come into being? What does this mean? ____

Analysis shows that shadows, eclipses and eclipses are all formed by the straight-line propagation of light. Therefore, four common phenomena in the problem can be explained by the principle of straight-line propagation of light. 1, as shown in Figure ①, you will see a very dark area on the wall, indicating that light travels in a straight line in the air. ② Change gestures. You will see the images of dogs, ducks, birds and other animals reflected on the wall, indicating that light travels along a straight line. (3) When the moon turns between the earth and the sun and is in the same straight line, the moon blocks the sunlight that hits the earth, and an eclipse occurs in the shadow area of the earth. (4) When the earth runs between the moon and the sun, and is in the same straight line, the earth blocks the sunlight that hits the moon, and the moon in the shadow cannot reflect the sunlight, thus forming an eclipse. Both solar and lunar eclipses are formed by the linear propagation of light. These four phenomena all illustrate the principle of straight-line propagation of light. When the light shines on an opaque object, a dark area beyond the light will be formed on the other side of the opaque object, and this dark area is the shadow. Let's think about it the other way around. If light does not travel in a straight line, it can bypass opaque objects and shine on the other side, so there will be no shadow. The appearance of shadows can only be explained by the propagation of light in a straight line.