# Human vision and function/Part 1: Perception of objects/Reading and questions about light and vision
## Concepts of light, light sense and vision

By the end of this reading, you students should have considered:
* What is light?
* What is light sense?
* What is vision?
* What is involved in perceiving an object or scene from the time you first look at it until you make your decision about what you have seen?

### What is light and what is light sense?

Light is a small component of the energy spectrum of electromagnetic radiation. It is that part of the spectrum which is visible to the human eye. It can stimulate the human eye because certain cells in the retina of the eye (photo-receptors) are sensitive to specific wavelengths of light. Very short waves of electromagnetic radiation, such as cosmic or ultra-violet rays do not stimulate vision either and remain unseen. Very long waves, such as radio or television waves, do not stimulate vision either and remain unseen. The wavelengths that comprise light (the visible spectrum) range approximately from 400 (violet) to 800 (red)nano-meters (10$^{-9}$ meters). This is a very small section of the electromagnetic radiation range of 10$^{-12}$ meters to 2000 meters. The speed of light is constant at 300,000 kilometers per second.
Theories of the behaviour of light are:
* Wave theory: where light emitted from a source travels in all directions away from the source in wave fronts.
* Quantum theory: where light is considered as separate packages of energy travelling in straight lines, known as photons.
Whilst two theories of light behaviour exist, each is acceptable for explaining specific phenomena and in fact, complement each other. The wave theory deals with the path of light travel and the quantum theory deals with the amount of energy. When light falls on an object it is absorbed and/or transmitted and/or reflected. The combination of these events is the sum total of what happens to the light. The varying degrees of each will depend upon the angle at which the light has struck the object, the nature of the object itself and the wavelength of the light.

### How do we measure the intensity of a light source? Why do we need to?

We don’t often consider standards for the measurement of light that surrounds us (ambient light). Most of us just know that if it’s getting too dark to see then we need to turn on a light source or move into an area of higher illumination. For example if we were outside reading a newspaper we may need to move out of a shadow to see the print more clearly. Believe it or not, occupational health and safety standards exist for the amount of light necessary for different environments and tasks. The standard unit of measurement of light intensity was the foot candle which literally represented the light from a single candle falling on a surface at a distance of 1 foot away. Modern terminology now uses the “lumen” rather than the foot candle. The intensity of light can be described as being inversely proportional to the distance of the surface from the light source. In simple terms this means that the closer the light is to the object, the brighter the object will appear and the further the light is from the object the duller it will appear!

### What is vision? What does the concept of vision involve from the stimulus to perception?

The concept of vision involved the process that begins with a stimulus. Generally speaking that stimulus is light. Light enters the eye and undergoes optical refraction (bending) as it passes through the structures of the eyeball that are transparent. It is transmitted to photo receptor cells within the retina and evokes a chemical reaction. This then evokes an electrical stimulus which travels along the optic nerves and the visual pathways: optic chiasm (crossing); optic tract; lateral geniculate nucleus (LGN); optic radiations to the primary visual cortex in the brain. The light rays form an image on the retina and the amount of electrical impulses transmitted to the brain are interpreted and we “see”. We “see” with our brain. Different parts of the brain, for example memory, are very involved in the process of perception. Visual awareness of the environment can be studied in separate categories although all functions are simultaneous:
* Light sense - differentiating between light and dark
* Colour sense - discrimination of different wavelength of light
* Form sense - discrimination of the parts of an image of the retina

### Concepts of light, light sense and vision playlist

Take a look at this YouTube playlist to consolidate your understanding of light.