What is Light ?

Understanding light (to explain vision and how the eye works): This follows the page introducing the anatomy of the eye.

Wavelengths and Colours:

Light is a form of energy.

Specifically, light is a general term that is commonly used to refer to the "visible spectrum", i.e. the range of wavelengths (sometimes referred to by their corresponding frequencies) that, together, form the visible part of the electromagnetic spectrum. "Visible" means something that can be seen using the eye, as opposed to "invisible" things that cannot be seen !

The visible part of the electromagnetic spectrum ranges (in wavelengths) from 380 nm to 750 nm.
Each wavelength corresponds to light of one colour so the "visible spectrum" range of wavelengths would look something like the block below if each wavelength appeared only once and they were all lined-up in order of increasing wavelength.

Short(er) Wavelengths High(er) Frequencies	Long(er) Wavelengths Short(er) Frequencies
Above : Colours of the "Visible Spectrum"

The term "visible spectrum" means the range of electromagnetic energy that most people (i.e. those with "normal" vision) can see with the naked eye. This is just a small part of a much wider range of energies, many of which we cannot see but some of which are used in other ways, e.g. Radio Waves, Microwaves, Ultra Violet (UV) Light, Infra-Red (IR) Radiation, X-Rays, and other wavelengths such as those used in various types of scanning and imaging equipment. In general, these other types of electromagnetic energy have wavelengths that are either less than 380 nm or more than 750 nm. (One nm = 10^-9m)

What is "White Light" ?

If light only exists as single wavelengths corresponding to the colours in the range shown above, what is "white light" ?

"White Light" is the way humans perceive and refer to our experience of receiving an approx. equal quantity of all the wavelengths (i.e. colours) in the visible spectrum. This explains why there are so many different "shades of white". That is, when we see "white" we are receiving all the colours in approximately equal amounts - but only approximately. The combination of wavelengths (colours) received by our eyes usually contains a bit more of some wavelengths (colours) than of others, hence some "whites" can appear to be slightly "yellow", some slightly "blue", and so on.

Just as white is not a "spectral colour", neither is black or grey.
White is the way we perceive an approx. equal presence of all colours and black is the absence of light/colour such that all colours/wavelengths are (equally) lacking. Shades of grey also correspond to approximately the same energy from a wide range of wavelengths (colours) of light in the visible range, but in decreasing quantities of light energy along a scale from:

• White - Lots of "light energy" in the form of approximately equal amounts of light of each (or most) of the many different wavelengths of "visible light", corresponding to different "colours", reaching the eye/brain, to
• Black - No "light energy", that is an imperceptibly small quantity of "light energy", if non-zero then in approximately equal amounts of the range of wavelengths of "visible light", corresponding to different "colours" reaching the eye/brain.

Propagation of Light:

"Propagation" is a word that is sometimes used in scientific contexts to mean "travel" or "movement".
For example, the term "propagation" is used in optics, e.g. in the context of light energy and other forms of energy, e.g. beams of X-rays, and other electromagnetic energy that can be described as moving in "waves". It is mentioned here to introduce this frequently-used scientific word. In simple terms, it is sufficient to describe how light "travels".

Light generally travels through air in straight lines, only changing direction when it is reflected, scattered, or passes from one type of substance (called a "medium") to another. For example, light changes direction slightly when it moves from air into water, or from air into glass, or vice-versa. This change in the direction of travel of light is due to refraction, which is explained later.

Note that there are some situations in which light travels in curves rather than in straight lines - as explained by the physics of diffraction and interference. However, for the simple cases of describing image formation within the eye and the manifestation and correction of short-sight and long-sight, it is sufficient to think of rays of light traveling through any one medium, such as air or water, in a series of straight lines.

What happens when light reaches the surface of an object ?

Light travels (propagates) from many different sources, including from large and powerful sources of illumination such as the sun or the main lights in a room. It is also reflected and scattered from many and various objects that are not sources of light - meaning that the light did not start at that object but simply reached it from somewhere else, then left it again - often in a different direction.

Eventually light reaches the surfaces of physical objects, e.g. it could come from a lamp in a light fitting in the ceiling of a room (the "source" of the light), propagate through through the air in the room (a "medium" throught which the light passes) until it reaches a surface such as a table or other item of furniture.

When light reaches an object it can do one or some combination of absorption, reflection, scattering or refraction.

Next : What does light do when it reaches an object (e.g. something in a room) ?