When white light, which contains all the colors of the visible spectrum, shines on a green leaf, we see the leaf as green. This is because the pigment chlorophyll in the leaf absorbs certain colors of light and reflects green light to our eyes. Understanding why this happens requires an examination of the properties of light and the interaction between light and matter.
The Nature of Light
Light can be described as both a wave and a particle. As a wave, light has a wavelength, which determines its color. The visible spectrum of light consists of wavelengths ranging from about 400-700 nanometers. Violet and blue light have shorter wavelengths, while red and orange light have longer wavelengths. Green light lies near the middle of the visible spectrum, with a wavelength of around 500-570 nm.
When light interacts with matter, it can be transmitted, absorbed, or reflected. Transparent materials like glass transmit most visible light. Opaque objects, on the other hand, both reflect light and absorb certain wavelengths. The wavelengths that are reflected determine what color our eyes perceive the object to be.
Pigments and Light Absorption
The green color of leaves is due to the presence of chlorophyll, a pigment found in plant cells. The purpose of chlorophyll is to absorb light energy which is used in photosynthesis. There are several types of chlorophyll, but chlorophyll a and chlorophyll b are the most common in higher plants.
Chlorophyll absorbs strongly in the violet, blue, and red regions of the spectrum. The absorption peaks for chlorophyll a are at 430 nm and 662 nm, while chlorophyll b has peaks at 453 nm and 642 nm. The green and yellow wavelengths are the least absorbed by chlorophyll and are reflected back to our eyes, causing us to see the leaf as green.
| Pigment Type | Absorption Peaks |
|---|---|
| Chlorophyll a | 430 nm, 662 nm |
| Chlorophyll b | 453 nm, 642 nm |
Interaction of Light with a Green Leaf
When white light shines on a leaf, the interaction of light can be categorized into three main processes:
Reflection
Some of the light that hits the leaf is immediately reflected off the leaf’s surface. The reflected wavelengths are what give the leaf its color. The cell structure and chlorophyll pigments reflect the green and yellow wavelengths.
Transmission
Some of the light passes through the thin translucent layer of the leaf. This transmitted light is typically the green wavelengths that are reflected by deeper cell layers.
Absorption
The key process is absorption. The chlorophyll and other pigments in the leaf cells absorb strongly in the violet, blue, and red wavelengths. The blue and violet wavelengths have shorter wavelengths and higher energies, while the reds have longer wavelengths.
This selective absorption of high and low wavelength light allows the plant to harness the optimum light energy for photosynthesis. The green and yellow wavelengths are just right in the middle of the spectrum, so they are transmitted and reflected, allowing us to see the leaf as green.
Other Leaf Pigments
In addition to chlorophyll, plant leaves contain other light-absorbing pigments. These include:
– Carotenoids – These absorb in the short wavelength blue region and reflect yellow and orange light. Carotenoids are responsible for the bright autumn colors.
– Anthocyanins – This pigment absorbs green and reflects red, creating the red autumn leaf colors.
– Tannins – Tannins absorb a wide range of wavelengths, producing brown colors.
While chlorophyll is the dominant pigment in leaves, the relative amounts of the other pigments affect the exact shade of green we see. The balance of pigments is influenced by plant genetics and environmental factors.
Conclusion
When white light containing the full visible spectrum shines on a leaf, the green color we see is due to the selective absorption by the leaf pigment chlorophyll of violet, blue, and red light. The middle wavelengths of green and yellow light are reflected and transmitted to our eyes. This gives the leaf its characteristic green color, which allows just the right light energies to be absorbed for photosynthesis. The interaction of light with specific pigments in materials produces the amazing diversity of colors we see in the natural world.