When light reaches the surface of an object
This explanation of the basic physics of light continues from the page introduction to light.
Light reaches objects from many different sources
- e.g. 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 surrounding objects e.g. reflection from mirrors, windows, buildings, the sea, lakes or ponds.
In general, when light (of any colour, or a range of wavelengths) reaches an object it can do one or
a combination of:
- Absorption
- Reflection
- Scattering
- Refraction
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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. |
In general, dark objects are move likely to absorb light energy, while objects that are light (in colour) are more likely to reflect or scatter the light energy they receive. |
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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
one 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 !) |
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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. |
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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.) |
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So
- Light reflects from
some surfaces, scatters from other surfaces, and is absorbed by some (dark non-shiny) surfaces.
- In the cases of objects that light can pass through, e.g. glass or water, light may be refracted at the surface of the object. Not necessarily all of the light is refracted though - a proportion of the light may be reflected, and some may be scattered or absorbed, especially if the surface is dirty or textured e.g. patterned or "frosted" glass.
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What determines which of these possibilities
will apply in any particular situation ?
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. polarization
states) that are more complicated to explain but
are also relevant to some aspects of vision (e.g.
explaining polarizing sunglasses). These are omitted
from these introductory pages.
In simple terms, the same object, e.g. a cube, is more like to:
- Absorb light - if it has an opaque matt (non-shiny) black surface
- Scatter light - if it has an opaque matt (non-shiny) white surface
- Reflect light - if it has a shiny finish e.g. mirror surface
- Refract light - if it is transparent to the wavelength of light that reaches its surface (e.g. if it consists of colourless glass and is illuminated by visible light e.g. a green laser beam) and the light reaches the surface of the object within a certain range of angles.
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Diagrams to show the difference between light being
reflected from an object, and
light being scattered from/by
an object ?
Remember the diagrams used to show reflection, compared with scattering:
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Reflection follows
the "Law of Reflection", which
is:
the angle of incidence (io)
= the angle of reflection (ro)
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Light scattered
at a surface travels away from it in
range of directions - whose profile
depends on many factors, not a single
law.
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Reflection vs. Scattering:
When light is reflected at a surface, it leaves that surface
in a specific direction (according
to the Law of Reflection).
When light is scattered at a surface, it leaves that surface
in very many different directions.
An example of a reflecting surface is a high-quality mirror.
An example of a scattering surface is a sheet of good quality matt white
paper.
In the real world (as opposed to
in scientific theory), most objects
are mostly-reflective, mostly-scattering,
or mostly-absorbing - but
some proportion of incident light
may behave in the other ways.
Most light-coloured objects around
us at home, at school, and in offices
are mostly-scattering. |
So, during daylight
or in an illuminated area, light bounces off most
objects - predominately by scattering, but in
some cases also by reflection.
It is necessary to know the above in order to understand
the eye and vision because eyesight, which is also called sight, seeing, vision and visual perception, is the perception by humans (and other animals) of light received by the organism's eye(s) from objects
in a scene, sensed via the eyes, then eventually processed by the brain.
NEXT : Read about What is eyesight ?
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