What are colors

Science in dialogue

What is color and how is it created? For example, why is a tomato red?

Strictly speaking, there is no color in nature, only electromagnetic radiation. Their visible part is called light and lies in the wavelength range from around 400 to 750 nanometers. Color is a subjective impression. It arises when our visual system processes the sensory stimulus that is triggered by light of different wavelengths on the retina of the eye.

The sum of all the wavelengths present in the light of the sun mix to give the impression of white light. The different colors of objects are mainly due to two phenomena: On the one hand, the emission and, on the other hand, the reflection of light.

We perceive colors with certain sensory cells on the retina, the so-called cones. There are three types, each of which has its maximum sensitivity in the areas of red, green and blue.

The light hitting the retina causes - depending on the frequency - an excitation of the different types of cones. The combination of the stimuli leads to a specific color impression after processing by the downstream nerve cells and by the brain.

Some objects emit light, such as stars or glowing metal. Depending on the material and temperature, they can shine in different colors.

In the case of bodies that do not illuminate themselves, the color "arises" because they are illuminated by light. Some parts of the light are then reflected by the illuminated bodies, others are absorbed. Which proportions these are in each case depends on the chemical properties of the material. During absorption, the electrons in the illuminated objects are excited by light of certain wavelengths and give off the energy again in the form of heat or other invisible electromagnetic radiation. The color impression therefore only arises from the portion of the light that is reflected.

The red color of the tomato is caused by the red pigment lycopene, which belongs to the group of carotenoids. This absorbs the green part of the light. The remaining, reflected parts hit the retina and our visual system processes the stimulus into the sensory impression red.

The question was answered by Dr. Dirk Schanzenbach, Institute for Chemistry at the University of Potsdam.