The eye's basic function is to catch and focus light onto the back of the eye, where there are sensory receptors to convert the light energy to neural signals which are relayed to the brain, allowing us to interpret images.
The visible spectrum
Because of the immense speed of light, the information that the brain captures is almost instantaneous. Humans are able to detect only a small portion of the electromagnetic spectrum. Light or visible radiation ranges from about 380 nm to 700 nm in terms of wavelength. Other wavelengths are not visible because they are not transmitted by the ocular media nor are they absorbed by the photopigment of the eye.
Transduction is carried out by light sensitive receptors in the form of photoreceptors residing in the retina. The photoreceptors comprises of 120 million rods and 6 million cones. Cones are concentrated in a region of depression in the retina that is slightly above the blind spot. This depression is termed the fovea. It provides the best spatial resolution for vision. Rods reside mainly outside of the fovea.
1 million ganglion cells carry information from the retina to the brain. In the fovea, the convergence factor is 1:1. In other words, each individual photoreceptor is connected to an individual ganglion cell. For photoreceptors in the periphery, convergence factor is 126:1, meaning many photoreceptors share one ganglion cell. This explains why periphery visual acuity deteriorates rapidly.
Colour and night vision
Cones provide for colour vision and function under high light intensity. Rods allow for monochrome vision and function under dim light. When we move from the sunny outdoors into a dark place like a cinema, our eyes do not adjust immediately. This is because under the intense outdoor sunlight, our rods are bleached. When we move into a dark place, it takes some time for the rods to recover.
For people who work in dark places like a photo studio, red or blue light is often used to illuminate objects. This is because rods reflect red and blue light and hence do not get bleached, allowing for preservation of night vision. However, rods absorb green light and therefore, green light cannot be used.