Overview of the eye
Figure 1. Anatomy of the eye. Note that the light-sensitive rods and cones are located on the retina.
Figure 1 shows a diagram of the eye. Light from our environment enters the eye through the pupil, is focussed by the lens, travels through the eye and is detected by special light-sensitive cells called rods and cones, which are located at the back of the eye on the retina.
The light from our environment is decoded into the objects that we see via a very complex process. The decoding begins within our eye itself in the retina, and the partially decoded message gets sent to our brain via the optic nerve, for further processing of the visual signals.
Figure 2. The pupil contracts when we experience high levels of illumination (left) and dilates when we experience low levels of illumination, to allow more light into the eye (right).
The way our visual system decodes information from light depends on how much light is entering our eye. When we are in well lit conditions, such as outside on a sunny day, or inside with all of the lights on, our cones receive the light rays and begin the visual process. Our pupils also contract to limit the amount of light entering our eye and not overwhelm our visual system. When we are in low levels of illumination, such as outside at night, or inside in a dark room, our rods receive the light and begin the visual process. In this instance our pupils dilate to allow as much light as possible into our eyes. When we experience intermediate levels of light such as outside at dusk, or when the inside lights are somewhat dim, both our cones and rods can receive light and begin the visual process.
When and how do we perceive in colour? This happens when we experience high levels of illumination in our environment, and our cones initiate the visual process. A common misconception about colour vision is that ‘our cones are used for colour vision, and our rods are used for black-and-white (or really greyscale) vision’. When we experience high levels of illumination, our cones do allow us to perceive the world in colour, however, they also allow us to perceive greyscales at the same time. When we experience very low levels of illumination, only our rods can absorb light, and they do not have the capability to allow us to perceive colour. In this case our vision is limited to perceiving greyscales. The Lights on and off exercise explores this idea, which is discussed further in So how do we see all the colours in the world?
