1. Generally speaking, shadows obtained on a plane (ground or wall, etc.). When an object is illuminated by light, it is called the projection of the object. As long as there is light and there is an object illuminated by light, there is a shadow. Sunlight can be considered to be parallel. When this light shines on an object, the resulting projection is called parallel projection. From this we can draw two conclusions:

When objects with the same height (1) are placed vertically on the ground, as shown in figure 1, their shadow lengths in the sun are the same.

(2) When objects of equal length are placed parallel to the ground, as shown in Figure 2, their shadows in the sun are equal to each other, and the shadow length is equal to the length of the object itself.

2.① At different times, the direction and size of the shadow of the same object may be different. At different times, the size and direction of the shadow of objects under the sun are changing. As far as the northern hemisphere is concerned, from morning till night, the direction of object shadow is: west → northwest → north → northeast → east, and the shadow length also changes from long to short and then to long.

② At the same time, the height of different objects is proportional to the shadow growth.

In other words, the height of a tall object, such as the height of a flagpole, can be calculated by using the above relationship.

Note: When calculating the height of an object using the shadow length, it is important to measure the shadow length of two objects at the same time.

Key point explanation:

1. Parallel projection is a kind of object projection, which is produced under the irradiation of parallel light. Using parallel projection knowledge to solve problems, we should distinguish between different moments and the same moment.

2. The parallel lines between the object and the corresponding points on the shadow indicate parallel light.