Understanding Light (To Explain How the Eye Works)

This follows the page introducing the anatomy of the eye.

Wavelengths and Colours:

Light is a form of energy. More specifically, light is a general term commonly used to refer to the "visible spectrum", which is the range of wavelengths (and their corresponding frequencies) that, together, form the visible part of the electro-magnetic spectrum. This ranges (in wavelengths) from 380 nm to 750 nm. Each wavelength corresponds to light of one particular 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.

Above: Colours of the "Visible Spectrum".

The term "visible spectrum" means the range of electro-magnetic 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 many different types of modern scanning equipment.

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 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 amount of each colour, but in decreasing amounts along a scale from white (lots of "light" energy in the form of many different wavelengths, corresponding to different "colours", reaching the eye/brain) to black (no "light" energy, that is an imperceptibly small quantity of "light" energy in the form of many different wavelengths corresponding to different "colours" reaching the eye/brain).

Propagation of Light:

"Propagation" is a term used (in the context of light energy and also some other forms of energy, e.g. beams of X-rays, or sometimes other forms of energy that can be described as moving in "waves") to mean "movement" . "Propagation" is mentioned to introduce this frequently-used term but initially it is sufficient to describe how light "travels", which means the same thing.

Light generally travels through the air in straight lines, only changing direction when it 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 straight lines - as explained by the physics of diffraction and interference, but 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 reaches objects from many different sources - both from large and powerful sources of illumination such as the sun or the main lights in a room, and also by reflection or scattering from all the surrounding objects.

When light reaches an object it can do one, or some combination of, the following:

	Absorption	Light energy goes into the object itself. Because the light goes into the object rather than leaving the surface of the object - and then some of that light entering the eye - the object is not "seen" as very bright. Instead, it is perceived to be dark (meaning that little light is traveling from that object into the eye). However, the object might still be obvious to a viewer, e.g. a dull matt-black object would still be seen if observed on a clean white surface. In that case the contrast makes the presence of the dark object obvious.
	Reflection	Light reaches the surface of a very shiny object and "bounces" off the object in the same way as a hard ball would bounce off an even flat surface (e.g. as in the game of snooker). That is, when it is reflected light leaves the surface of an object at a particular angle relative to the angle from which it reached that surface. Law of Reflection: Angle of incidence = Angle of reflection (i.e. the same ONE angle !)
	Scatter	On reaching the surface of an object, light leaves that surface not in any one particular direction, but in many directions spread over a wide range of angles. This applies particularly to non highly-polished surfaces, such as paper, or walls painted matt white. Scatter is the most common of these possibilities when visible light is incident on ordinary everyday solid/opaque objects.
	Refraction	This is another case of light entering the object instead of leaving the surface of the object. Refraction only applies to objects that light can pass through, such as blocks of glass or plastic, windows, water, and spectacles. It is mentioned here for completeness. In the context of explaining how light reaches a person's eye from objects in the real world in front of him/her, refraction is less important that the other possibilities described above. (However, note that refraction plays a very important role in the eye/visual system for other reasons, such as focusing images onto the retina - and is therefore explained on other pages later in this section.)

To summarise:

Tell me more: What determines which of these possibilities apply to a situation (i.e. an illuminated object) ?

A solid opaque object absorbs, scatters, or reflects light (or some combination of these) depending on its physical properties, including the properties of the material the object is made or formed from.

Aspects of objects that influence the onward path taken by light reaching their surfaces include:

• the physical state of the object (solid, liquid, gas),
• the substance it is formed from (wood, rock, glass etc.),
• the texture of its surface (rough, polished, carved, etc.),
• the thickness of the object/material (thin sheet of ice, or huge iceberg), and even
• its colour.

Other factors that affect how much light is absorbed, reflected, and scattered concern the light itself and how it arrived at the surface of the object, such as:

• the wavelength (colour) of the light, and
• the angle at which it reaches the surface.

Light also has other properties (e.g. polarisation states) that are more complicated to explain but are also relevant to some aspects of vision (e.g. explaining polarising sunglasses). These are omitted from these introductory pages.

Summary of the difference between light being reflected from an object, and light being scattered from/by an object ?

Remember the diagrams used to summarise reflection, as compared with scattering:

Reflection follows the "Law of Reflection", which is: the angle of incidence (io) = the angle of reflection (ro)	Light scattered at a surface travels away from it in range of directions - whose profile depends on many factors, not a single law.

Reflection vs. Scattering:

So, during daylight or in an illuminated area, light bounces off most objects - predominately by scattering, but in some cases also by reflection.

So what ?

This information is necessary in order to understand the eye and vision, because:

When any amount or colour of light is present in a space (e.g. a room), it bounces around the space, from object to object.
As mentioned above, one can imagine this as a series of straight lines - which are often represented on diagrams as "rays", i.e. lines with arrows indicating the direction of travel of the light.

So,

Light that is either reflected from or scattered by a surface enables people to see that surface (object) because when light leaving the surface of the object enters the eye(s) and reaches the "screen" called the retina located at the back of the eye it causes signals to be transmitted to the brain. Those signals are then processed by the brain, generating the experiences we understand as seeing and our view of the world around us.

Because light is only reflected from a surface at the angle at which it arrived at that surface, reflections may only reach the eyes from a very limited range of angles. However, light scattered from surfaces travels in many more directions so is far more likely to reach and enter the eyes of people in the same area as the object. Most of our visual perception of the world around us is therefore due to scattered light.

How do we see the light coming from the objects around us ?

People only "see" (any object) when light from that object passes into at least one eye in such a way as to reach the back of the eye where signals corresponding to that light are sent to the brain and processed.

This occurs when some of the light propagating from objects around us reaches and enters our eyes - which dynamically adjust as necessary to produce (usually!) clear pictures called "images" of the objects we look at. These images are formed on a "screen" called the retina at the back of each eye.

Good eyesight/vision requires that the images formed on the retina are clear, sharp images.

The next page explains the process of image formation within the eye.