Why is Grass Green?
Grass, along with most vegetation may be green to prevent overheating by sunlight, which is also primarily green. Answering this deceptively simple question involves chemistry, biology and physics.
Grass appears green because of a precise interplay of chemistry, biology and physics, centered on how plant cells interact with light. The vibrant hue stems from molecular properties of the pigments in blades of grass, especially chlorophyll. Light from the Sun spans the visible spectrum of wavelengths, roughly between 400 and 700 nanometers, going from violet to red, or all the colours of a rainbow. The dominant pigment in blades of grass is chlorophyll, a complex molecule found in the chloroplasts of plant cells. Chlorophyll absorbs light primarily in blue (400-500 nm) and red (600-700 nm) regions of the spectrum. The green wavelengths (500-570) are largely unabsorbed, so they are reflected or transmitted, reaching the eyes of the observer in the characteristic colour.
The selective absorption by chlorophyll is an evolutionary adaptation with the molecular structure centered on a porphyrin ring with a magnesium ion, that allows it to capture photons efficiently for photosynthesis, the process by which plants convert light, carbon dioxide and water into oxygen and glucose. The blue and red wavelengths are energetically optimal for driving the chemical reactions in photosynthesis. Green light is less useful, and is reflected by plants. This reflection is why grass appears green under white light. The output from the Sun peaks in green frequencies and scientists have theorised that plants reflect the green light to prevent overheating and cell damage.
Two types of chlorophyll
Plants contain two types of chlorophyll, a and b. Chlorophyll a absorbs light around 430 and 663 nm while chlorophyll b absorbs light at 453 and 642 n. The slightly different absorption spectra broadens the range of light that grass can use, but both reflect light in green wavelengths, reinforcing the colour of the plant. Grass also has other pigments, including carotenoids that absorb blue light and reflect yellows or oranges. Carotenoids are present in much smaller quantities than chlorophyll, and are typically masked by the chlorophyll. However, they become visible when chlorophyll breaks down, just before the winter months. This is why ageing or dry grass appears yellow. Chlorophyll dominates in healthy grass, indicating its good health.
The green appearance is also enhanced by the physical structure of the green grass. The mesophyll layer of cells within a blade of grass scatters light, increasing the likelihood that the unabsorbed green wavelengths escape easily. Additionally, the waxy cuticle on the blades of grass can reflect light, amplifying the green hue, especially when the grass is wet or under direct sunlight. Chlorophyll is highly efficient and stable, and its dominance likely arose because early photosynthetic organisms thrived in oceans, where the green light penetrated to deeper depths. Grass, like other planets, inherited this trait from the marine ancestors. This is the primary reason that plants do not use other pigments to capture different wavelengths - mostly. Alternative pigments such as red algae exist in specific niches but are less common in terrestrial plants. Grass, however is green because it absorbs red and blue light to drive photosynthesis.