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.

The word light 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 human eye, as opposed to 'invisible' things that cannot be seen by the human visual system on its own*.

The visible part of the electromagnetic spectrum ranges (in wavelengths) from 380 nm to 750 nm.

Short(er) Wavelengths High(er) Frequencies	Long(er) Wavelengths Short(er) Frequencies
Above : Colours of the visible spectrum

So, the expression '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. 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' ?

Given that light only exists as single wavelengths corresponding to the colours in the range shown above, what is 'white light' ?

'White Light' or 'the colour white' is the way humans perceive and refer to our experience of receiving an approximately equal quantity of all the wavelengths (i.e. colours) in the visible spectrum from an area or object. We describe and refer to such objects or regions as 'white' e.g. a sheet of white paper, a bright white moon (in certain atmospheric conditions) and the colour of a dove. There are many different 'shades of white' because when we see the 'colour 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, slightly blue, slightly pink, and so on.

The statement 'white is not a spectral color' means that it does not correspond to a single wavelength in the visible spectrum, i.e. the colour range represented above. Instead, white is the way we perceive an approximately equal presence of all colours while 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' (luminance) 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' or an imperceptibly small quantity of 'light energy' (luminance), if non-zero then in approximately equal amounts of the range of wavelengths of 'visible light', corresponding to different 'colours' reaching the eye / brain.

Brown is also a non-spectral colour, meaning that none of the many shades of brown correspond to specific electromagnetic wavelengths. Instead, 'brown' is the word used to refer to the human visual experience of detecting a particular range of combinations of wavelengths (colours of light), typically relatively small 'amounts' (luminance) of shades of yellow, orange and red. Due to brown being the experience of detecting a combination of wavelengths (colours) of light, brown is sometimes referred to as a composite colour. Although this may also apply to many other shades of colours e.g. as used to describe the visual experience of looking at various pigments, paints and surfaces, brown is usually the example given in textbooks, perhaps because it is so commonly experienced in the natural world.

Propagation of Light:

The word 'propagation' is sometimes used in scientific contexts to mean 'travel' or 'movement'. It is used in optics, which is the study of the physics of light and other forms of electromagnetic energy, in the context of describing energy moving ('propagating') in 'waves'.

Light is generally considered to travel 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.

Note: 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 (originate from) that object but arrived at it from somewhere else, then left it again, perhaps 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 the air in the room (a 'medium' through 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) ?

* without the use of specifically designed equipment such as used to form images using electromagnetic radiation outside of the visible spectrum and / or in very low-light conditions e.g. at night or in enclosed spaces.