When the sun just goes down, the light of the street lamp is not strong enough, and our shadow will be longer. This is because the sun is below the horizon, and when the light hits the ground, it will form a larger angle, making the length of the shadow relatively long. At this time, our shadow may extend far away, even beyond our own height.
As time goes on, the sun gradually goes down and the angle of light gradually becomes smaller. At this time, our shadows will gradually become shorter. When the sun is close to the horizon, the angle of light is the smallest and the length of shadow is the shortest. At this time, our shadow may be only half of our height or even shorter.
When the sun goes down completely, the light from street lamps becomes our main light source. At this point, our shadows will become very blurred or disappear. This is because the light of the street lamp is emitted from all directions, and there is no clear direction of the light source, so it is impossible to form a clear shadow.
Besides the change of sun position and light angle, other factors will also affect the change of shadow length. For example, the occlusion of clouds, buildings or trees will affect the illumination of light, and then affect the length of shadows.
Generally speaking, walking under a street lamp at night, the length of the shadow will change with the passage of time. From the long shadow when the sun just goes down, to the short shadow when the sun approaches the horizon, and then to the blurred or disappeared shadow after the sun completely goes down, these changes are all caused by the changes in the angle and intensity of light. This change also brings people a sense of time flow, which makes people think and feel involuntarily.