Why is the blackboard green

The characteristic feature of any classroom is the blackboard on which text is written and drawings are made with the help of calcium sulphate chalk. These boards are typically made of sheets of dark grey or black slate stone. In recent years, green colored boards have largely replaced blackboards. Studies reveal that green color relaxes the eyes and prevents fatigue. Furthermore, the human eye is highly sensitive to the green color, and for this reason, the use of green boards has extensively increased.

The retina of the eye has two types of photoreceptor cells, cones and rods. There are approximately 120 million rods in the eye, which are responsible for scotopic vision — they help the eye to perceive light and darkness. The cells in the retina that recognize color are called cones. There are 6 to 7 million cones that are segregated into three distinct categories: red cones, green cones and blue cones. Although the cones are less sensitive to the light, but they are responsible for all the high resolution vision.

The visual center of the eye is fovea centralis, which is a slight depression on the retina, less than 2mm in diameter. This region is tightly packed with red and green cones. The blue cones, which have the highest sensitivity, lie outside the fovea centralis. Rods are also not found in this region. The two kinds of photoreceptors of the eye interact with one another to interpret the color and contrast of an object.
The eye can detect only visible light that consists of wavelengths between 390 nm and 780 nm. The longest wavelength is perceived by the human eye as red color and the shortest wavelength is perceived as violent color. In between other colors as orange, yellow, green and blue come. When the light enters the pupil and falls on the retina, both the rods and cones try to interpret it.
While the rods decode the intensity of the light, cones recognize the color and contrast. The chemical reaction occurs as the result of the interaction between the light and the rods or cones. The resultant electrical impulses are sent to the brain, which in its turn helps the eye and body to respond to the light.

Eye anatomy


Under normal circumstances, the red cones are activated not only by the red wavelength of light, but also by yellow and orange wavelengths. Similarly, the green cones are activated by green, blue and yellow wavelengths of light. The sensitivity of the eye is highest between the wavelengths 507nm and 555nm. This is where the green color resides. Thus, to get the maximum benefit, the color of the blackboard is green.