Quest Korner

Why are fruits, vegetables, and flowers differently colored?

We see several plants around us. They are the most important natural resource of the planet Earth. They play vital role in our life. Plants are the backbone of all life on Earth. They are the essential resource for human well-being. Plants produce tasty fruits, vegetables and scented flowers. The specialty of the plant world is that they are all vividly colored. Why?

All green plants, some vegetables like ladies finger, Broccoli etc are green in colour because of the presence of green colored pigments called Chlorophyll. Carrots are orange in color, ripe bananas, mangoes are yellow in color. Mangoes of different varieties are differently colored. Flowers are not only scented but also colored vividly. Grass is green. Plant world is amazing us with beautiful colors. What causes such beautiful colors in the plant world? Of course pigments are responsible for the beautiful colors we see in the world of plants.  Some of the pigments are:

1. Chlorophyll is green in color and is responsible for the green color of the foliage and leaves.

2. Carotenes and Xanthophylls are found in carrots and mangoes, responsible for ripe yellow and orange color.

3. Certain flavonoids like Anthocyanins are present in egg plant, berries and citrus fruits.

4.  Betalains, another plant pigment consists of two sub groups, red-violet (betacyanin) and yellow to orange (betaxanthin) pigments.  Betacyanin give rise to deep red color.

Thus, pigments play a vital role in coloring the plant world. Light also contributes to it. Let us see the role played by light in coloring the fruits, vegetables and flowers.

When we use the term "light," we are referring to a type of electromagnetic wave that stimulates the retina of our eyes. Electromagnetic waves exist in a vast range of wavelengths; our eyes are sensitive to only a very narrow band. Visible light spectrum is the narrow band of wavelengths the humans see. This visible light region consists of a spectrum of wavelengths that range from 7 x 10^-7 meter to 4 x 10^-7 meter. This narrow band of visible light is known as VIBGYOR.

Sunlight is white light and visible light. It is the mixture of seven colors. The separation of visible light into its different colors is known as dispersion. When all the wavelengths of the visible light spectrum strike our eyes at the same time, white is perceived.  The sensation of white is not the result of a single color of light. Rather, the sensation of white is the result of a mixture of two or more colors of light. White is the combination of all the colors of the visible light spectrum.

Each color is a characteristic of a distinct wavelength; and different wavelengths of light waves will bend varying amounts upon passage through a prism. The red wavelengths of light are the longer wavelengths and the violet wavelengths of light are the shorter wavelengths. Between red and violet, there is a continuous range or spectrum of wavelengths. 

The colors of the objects that we see are largely due to the way those objects interact with light and ultimately reflect or transmit it to our eyes. The color of an object is not actually within the object itself. Rather, the color is in the light that shines upon it and is ultimately reflected or transmitted to our eyes. We know that the visible light spectrum consists of a range of frequencies, each of which corresponds to a specific color. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light that strikes the object and becomes reflected or transmitted to our eyes will contribute to the color appearance of that object. So the color is not in the object itself, but in the light that strikes the object and ultimately reaches our eye. The only role that the object plays is that it might contain atoms/molecules  capable of selectively absorbing one or more frequencies of the visible light that shine upon it.

The pigment carotene present in carrots, oranges, mangoes absorbs blue and indigo light of sunlight and transmits yellow and orange light.

Grass and lush foliage is green in color because the green pigment chlorophyll transmits green light and absorbs rest of the colored light.
Lycopene is the pigment responsible for red color of tomatoes, guava, melons and

grapefruits. 

When white light falls on these fruits all the colors are absorbed except red and red 

light is transmitted.

Bananas and Mangoes look yellow and golden as orange and yellow colors are reflected. The rest of the colors are absorbed by bananas.

How do we see color?

Human eye and the brain together translate light into color. The surface of an object reflects some colors and absorbs all the others. Therefore, we perceive only the reflected colors. Retina of the eye is covered by millions of light sensitive cells. Some are shaped like rods and some are like cones. These receptors process the light into nerve impulses and pass them along to the cortex of the brain via the optic nerve.

Rods are concentrated around the edge of the retina, while cones are concentrated in the middle of the retina. There are three types of cone-shaped cells, each sensitive to the long, medium or short wavelengths of light. These cells, working in combination with connecting nerve cells, give the brain enough information to interpret and name colors.

Each individual wavelength within the spectrum of visible light wavelengths is representative of a particular color. That is, when light of that particular wavelength strikes the retina of our eye, we perceive that specific color sensation. That is how we see  different colors in the plant life.

*************************************************************