A cast shadow is the shadow an object throws onto the surface behind or beneath it. It is one of the strongest clues that an object sits in a real space.
Key idea
A cast shadow falls away from the light. It touches the object where the object touches the surface, and it is darkest and sharpest there, getting softer as it goes out.
Why it works
The object blocks light, so the shadow lies on the far side of the object from the light. Right at the contact, almost no light can reach, so that spot is darkest. Farther out, light from the edges of the source sneaks in around the object, which softens the shadow's edge. When the light is low, as in the evening, shadows stretch long.
Worked example: An evening walk
At sunset the sun is low in the west. Which way do the shadows of trees point, and are they long or short?
- Shadows point away from the light: toward the east.
- A low light hits objects from the side, so their shadows stretch out.
Answer: Toward the east, and long.
Common mistake
People leave a gap between an object and its shadow, so the object seems to float. The shadow touches where the object touches. Put the darkest dark right at the contact, then let the shadow lighten as it moves away.