What is Light ?
Understanding light (to explain vision and how the eye works): This follows the page introducing
the anatomy of the eye.
Why Understand Light ?
In order to appreciate how the eye works,
what the different parts of the eye do,
and what happens when there are problems with vision,
it helps to know a few simple things about the nature of light, including:
- What Is Light (wavelengths and
colours (i.e. "colours" that don't correspond to a specific wavelength of light, incl. white,
black and greys)
of light (usually in straight
lines while traveling through any one
medium, e.g. air.)
of light at surfaces (absorption,
reflection, scattering or refraction)
- explain why light travels away from
most illuminated objects in many different
directions - hence in most cases some
light from all objects in a person's "field
of view" reaches his/her eyes - exceptions
for very dark objects.
Wavelengths and Colours:
is a form of energy.
is a general term that is commonly used to refer to the
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.
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
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" ?
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
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
- 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,
- 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
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".
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) ?