Why are shadows black

Science in dialogue

Why is the shadow black?

An object is usually perceived as black when it reflects significantly less light than its immediate surroundings. This is exactly the case with shadows: because an object on which a shadow falls reflects significantly less light than its surroundings, the shadow usually looks black.

This does not depend on the absolute “amount” of light that the object reflects. The decisive factor is the relationship between the light reflected by an object and the brightness in its immediate vicinity. For example, a “white” sheet of paper looks white in a dark room, and a “black” sweater stays black in bright sunlight. This is true even though a lot more light is reflected into the eye from the dark sweater in the sun than from the light paper in a dark room.

Such contextual effects - i.e. the influence of the environment on the perception of an object - also occur with colors. Color is neither a property of objects nor a property of light. Light from a light source - e.g. the sun - is generally composed of different wavelengths. One speaks of the spectrum of light. Depending on their physical properties, illuminated objects reflect different parts of the spectrum. If the reflected light reaches the human eye, it activates light-sensitive cells on the retina. These cells are called cones. The eye has three different types of cones. They are excited in different ways by light of different wavelengths. This creates the perception of different colors.

However, the fact that color is not a purely physical phenomenon is shown by the fact that the color of an object is not determined solely by the spectrum of the reflected light. It depends essentially on the lighting of its surroundings.

In the phenomenon of simultaneous contrast, for example, the color of a circle differs depending on whether it is presented in a white or in a green environment. In the first case the circle appears gray, in the latter it appears reddish. Its color is thus shifted to the opposite color of the surroundings. The spectrum of the light that is reflected from the circle has not changed.

The “colored shadows” represent a similar phenomenon. The shadows take on the opposite color of the surroundings due to colored lighting. In the experiment, colored shadows can be created by illuminating an object with two light sources. One light source emits “white” and the other, for example, “blue” light. The object then casts two shadows on an area placed behind the object. The shadow illuminated by the “blue” light source appears blue, while the shadow illuminated by the “white” light source appears yellow rather than white.
  
The question was answered by Martin Giesel and Prof. Karl Gegenfurtner, Department of Psychology at the Justus Liebig University in Giessen.