Ever wondered why you can never seem to see things clearly at night? Or why the bright colors during the daylight fade into hues of blue and brown as the sun sets? Or why you find it really difficult to focus on minute details at night even though you were able to see them clearly in broad daylight?
It all depends on the so-called Purkinje Effect, which dictates what the human eye can and cannot see in dim light. According to this effect, the eye seems to perceive the same things in different ways under different light conditions. Here’s a small explanation as to why this occurs.
The human eye is adapted to seeing things better under light of varying degrees of brightness. Be it sunlight, moonlight or even starlight, the human eye can change the size of the pupil to adjust to the brightness that is available at that moment. However, not just the pupil alone governs the functioning of the eye. Certain other visual systems control the way the eye works. And two systems in particular are responsible for this so called ‘color fade’ in the evenings and nights.
These two systems (aka two photoreceptors in retinas) are called the cones (color receptors) and rods (light receptors). The cones work best under bright light, can see color vividly and can focus on fine details effectively. The rods on the other hand are sensitive to light and can only see light and dark. They cannot see any colors or differentiate between them. The rods would need to borrow light from the other parts of the eye to adjust vision and so, cannot focus on fine details like the cones.
Now, try to understand what exactly happens when the lights dim and the colors start fading. The cones respond better to yellowish light and so find it difficult to function under the receding light, passing on the responsibility of vision to the rods.
The rods in turn respond better to blue/green light and do their work of providing information about the shape and contrast to the brain, sans any colors. This explains the monochrome vision experienced (mostly bluish/grayish hues) at nightfall and afterwards.
