Advanced Physiology

Advanced Physiology

a) How does the eye determine that the light is red?

The cross section of eye has three layers namely Sclera, Choroid and Retina. The image we see straightly falls on the Retinal layer. In the retinal layer we have two types of cells namely Rods and Cones. We have approximately 125 million rods. They are spread throughout the peripheral retina and function best in dim lighting. The rods are responsible for peripheral and night vision. (Fernald 2006: 1914-1918)

Cones are especially important for vision in bright lighting situations, for acute vision (i.e. receiving sharp detailed images), and for colour vision.

There are thought to be three distinct types of cones, each type being sensitive to a specific band of wavelengths of light (usually described in terms of the primary colour red). Perception of other colours is explained in terms of combinations of the three bands of wavelengths detected by the three types of visual cones. Each human eye contains approx. 6-7 million cones - compared with approx. 125 million rods.

Red cones detect red light, blue cones detect blue light, and green cones detect green light. The different ratios of these cells which are stimulated by light allow us to construct all the colours that wee see (yellow is a combination of green and red, for example). Colour is our perception of different wavelengths of light. Light visible to humans ranges in wavelength form 380 nanometers (nm) for violet light to 760 nm for red light. Red-sensitive cones are most stimulated by light in the red to yellow range. Colour vision in humans is based on the additive colour theory. This theory states that all perceivable colours can be made by mixing different amounts of red, green, and blue light, the primary colours of the additive colour system.

Additive colour theory explains what we see when we look at a luminant object such as a light bulb, a TV, or a computer monitor. An ordinary incandescent light bulb emits light over most of the visible spectrum, but is strongest in the red to yellow range. A standard "cool white" fluorescent bulb has a narrower emission profile than an incandescent bulb, producing more light in the green to blue range.

The screen of a TV or a computer monitor is made of millions of red, green, and blue phosphors closely packed together. When stimulated by a high voltage, groups of phosphors glow to create an image on the screen. Millions of colours can be created by combining phosphors of these three primary colours. (Vicki 2009: 20)

Human beings are so used to observing the physical world visually that we equate visual observation with reality. But this is not necessarily correct. The human eye has operating characteristics that limit the accuracy of visual observations. For example, everyone knows that we can only see light over the limited range from red to blue but we presume that there is no distortion of perception over this visual range. But this is definitely not ...