It was mentioned earlier in
this lesson that light reflects off surfaces in a very predictable manner - in
accordance with the law of reflection. Once a
normal to the surface at the point of incidence is drawn, the angle of
incidence can then be determined. The light ray will then reflect in such a
manner that the angle of incidence is equal to the angle of reflection. This
predictability concerning the reflection of light is applicable to the
reflection of light off of level (horizontal) surfaces, vertical surfaces,
angled surfaces, and even curved surfaces. As long as the normal (perpendicular
line to the surface) can be drawn at the point of incidence, the angle of
incidence can be measured and the direction of the reflected ray can be
determined. A series of incident rays and their corresponding reflected rays
are depicted in the diagram below. Each ray strikes a surface with a different
orientation; yet each ray reflects in accordance with the law of reflection.
|
The Law of Reflection is
Always Observed (regardless of the orientation of the surface) |
In physics class, the behavior of
light is often studied by observing its reflection off of plane (flat) mirrors.
Mirrors are typically smooth surfaces, even at the microscopic levels. As such,
they offer each individual ray of light the same surface orientation. But quite
obviously, mirrors are not the only types of objects which light reflects off
of. Most objects which reflect light are not smooth at the microscopic level.
Your clothing, the walls of most rooms, most flooring, skin, and even paper are
all rough when viewed at the microscopic level. The picture at the right
depicts a highly magnified, microscopic view of the surface of a sheet of paper.
Reflection off of
smooth surfaces such as mirrors or a calm body of water leads to a type of
reflection known as specular reflection. Reflection off of rough surfaces such as clothing, paper, and the
asphalt roadway leads to a type of reflection known as diffuse
reflection. Whether the surface is microscopically rough
or smooth has a tremendous impact upon the subsequent reflection of a beam of
light. The diagram below depicts two beams of light incident upon a rough and a
smooth surface.
A light beam can
be thought of as a bundle of individual light rays which are traveling parallel
to each other. Each individual light ray of the bundle follows the law of
reflection. If the bundle of light rays is incident upon a smooth surface, then
the light rays reflect and remain concentrated in a bundle upon leaving the
surface. On the other hand, if the surface is microscopically rough, the light
rays will reflect and diffuse in many different directions.
For each type of
reflection, each individual ray follows the law of reflection. However, the
roughness of the material means that each individual ray meets a surface which
has a different orientation. The normal line at the point of incidence is
different for different rays. Subsequently, when the individual rays reflect
off the rough surface according to the law of reflection, they scatter in
different directions. The result is that the rays of light are incident upon
the surface in a concentrated bundle and are diffused upon reflection. The
diagram below depicts this principle. Five incident rays (labeled A, B, C, D, and E) approach a surface. The normal line
(approximated) at each point of incidence is shown in black and labeled with an N. In each case, the law of reflection is followed, resulting in five
reflected rays (labeled A,, B,, C,, D,, and E,).
There are several
interesting applications of this distinction between specular and diffuse
reflection. One application pertains to the relative difficulty of night
driving on a wet asphalt roadway compared to a dry asphalt roadway. Most
drivers are aware of the fact that driving at night on a wet roadway results in
an annoying glare from oncoming headlights. The glare is the result of the
specular reflection of the beam of light from an oncoming car. Normally a
roadway would cause diffuse reflection due to its rough surface. But if the
surface is wet, water can fill in the crevices and smooth out the surface. Rays
of light from the beam of an oncoming car hit this smooth surface, undergo
specular reflection and remain concentrated in a beam. The driver perceives an
annoying glare caused by this concentrated beam of reflected light.
A second
application of the distinction between diffuse and specular reflection pertains
to the field of photography. Many people have witnessed in person or have seen
a photograph of a beautiful nature scene captured by a photographer who set up
the shot with a calm body of water in the foreground. The water (if calm)
provides for the specular reflection of light from the subject of the
photograph. Light from the subject can reach the camera lens directly or it can
take a longer path in which it reflects off the water before traveling to the
lens. Since the light reflecting off the water undergoes specular reflection,
the incident rays remain concentrated (instead of diffusing). The light is thus
able to travel together to the lens of the camera and produce an image (an
exact replica) of the subject which is strong enough to perceive in the
photograph. An example of such a photograph is shown below.
Photograph
of Mount Moran in the Grand TetonNational Park
in Wyoming - taken by Becky Henderson |
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1. If a bundle of
parallel incident rays undergoing diffuse reflection follow the law of
reflection, then why do they scatter in many different directions after
reflecting off a surface?
Each individual
ray strikes a surface which has a different orientation. Since the normal is
different for each ray of light, the direction of the reflected ray will also
be different.
2. Perhaps you
have observed magazines which have glossy pages. The usual microscopically
rough surface of paper has been filled in with a glossy substance to give the
pages of the magazine a smooth surface. Do you suppose that it would be easier
to read from rough pages or glossy pages? Explain your answer.
It is much
easier to read from rough pages which provide for diffuse reflection. Glossy
pages result in specular reflection and cause a glare. The reader typically
sees an image of the light bulb which illuminates the page. If you think about,
most magazines which use glossy pages are usually the type which people spend
more time viewing pictures than they do reading articles.