Not all colors in the light spectrum are visible to the human eye. Some wavelengths, such as infrared, are too long for humans to visually process. Others, such as ultraviolet, are too short. The shortest wavelength processed by the human eye is recognized as the color violet. Violet light, in conjunction with stronger blue light, contributes to plants' vegetative growth.
Wavelengths and Development
Green and yellow wavelengths affect plant growth the least. These colors are reflected from a plant, not absorbed. Red and blue wavelengths contribute the most to plant development. Red waves assist in the development of flowers. Blue light promotes bushy growth and is especially helpful for seedling development. Short-waved violet light is crowded out by strong blue light waves, limiting the effect of violet rays on plant growth.
Farmers and other plant growers have long noticed that plants bend toward the light. At first, it was thought this phenomenon may be due to wind rather than light. Further experimentation revealed that light was the cause, and the occurrence was called phototropism. In the mid-1800s scientist J.V. Sachs discovered that when combined, violet, indigo and blue light act similarly to white light when inducing a phototropic response. Red light waves do not cause phototropism.
The sun provides all the rays a plant needs for healthy growth. If located in a dark area of a home or during winter months, plants require supplemental lighting. For bushy growth, or for seedlings, choose fluorescent lights for their blue and violet light waves. Incandescent lights provide red waves that are beneficial for flowering plants. Ultraviolet lights do not assist in plant development.
Violet light and ultraviolet light are not one in the same. At less than 400 nanometers in length, ultraviolet light waves are shorter than violet wavelengths. Both are considered "bluer than blue light," but the human eye can see violet light. Violet rays are scattered throughout the atmosphere. We can see them through prisms of water (rainbows) and through pollutants in the atmosphere (sunsets).
The Earth's atmosphere protects us from ultraviolet light. UV rays cause sunburn and physical damage. Even though ultraviolet light is bluer than blue, this excess blueness will not help seedlings or promote growth. The same UV rays that kill bacteria can stunt a plant's growth. Scientists from the Japan Atomic Energy Research Institute have noted that UV light damages plant DNA and can create mutations in future generations of a plant.