Shadows appear when an opaque object blocks light from reaching a surface behind it.

The longer version

Light travels in straight lines, so anything solid placed in its path removes part of the beam and leaves a region where fewer photons land. The shape of that region is in effect a projection of the object, cast outward from the light source onto whatever surface happens to sit behind it.

Distance changes the result: moving an object closer to the surface sharpens the outline, while moving it away enlarges and softens it. Multiple lights produce multiple shadows at once, each from a different angle. On a cloudy day, light arrives from the whole sky and shadows nearly vanish.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.