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Of all the
electromagnetic waves, light is the only portion of waves
that can be detected by the human eye.
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Section
1. Luminous and illuminated bodies. |
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Light is
produced by a luminous body. A light bulb is a
luminous body that emits light in almost every direction. Light travels in straight lines
at 299,792,458 m per sec in vacuum. When light hits an
object, it is reflected. An illuminated body reflects light.
When a ray of light
reaches our eyes, the receptors in our eyes will produce
a different color sensation depending on the wavelength
of the light wave.
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Section
2. Luminous flux and illuminance. |
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The amount
of light produced per unit of time is called the luminous flux, P. The unit
for this is the lumen, lm. If we only want to know the
amount of light produced by a source on a specific object
(table, chair), we have to use the formula for illuminance, E. This
formula tells us the amount of light per meter square at
a certain distance.
E = P/4(pi)dē
Thus, the illuminance
is just the luminous flux divided by the area of a
sphere. Why a sphere? For example, lets put two objects
in different places, both of them two meters away from a
source of light.

As you can see the
illuminance on these objects is the same. This is because
almost every source of light emits light in all
directions and two objects within the same radius receive
the same amount of light.
However, objects that
are placed at different distances have different
illuminations.

The closer an object is
to a source of light, the more illuminated it will be.
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Section
3. Colors Red, green
and blue are known
as primary colors, because when they are added together
white light is formed.

By mixing primary colors in
pairs we obtain secondary
colors. Red and green
produce yellow. Blue and red produce magenta, and
blue and green produce cyan.
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Section
4. Reflection of light |
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If we draw a line perpendicular to a
surface, this line is the normal of the surface. When a
ray of light hits the surface of an object, part of the
light is reflected. If the ray of light is in angle with
the surface, then the angle between the incident ray and
the normal will be the same angle between the normal and
the reflected ray. 
They are not completely flat surfaces. When millions
of rays of light hit the rough surface of an object, they
are reflected in all directions. This is how we can see
illuminated objects.
How does a mirror reflect light?
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Section
5. Refraction of light |
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When a ray of light passes from one
medium to another, it bends. Depending of the new medium
the light will travel faster or slower. If the light
travels faster in the second medium, then this medium is
called the rarer medium. On
the other hand, the medium in which the light travels
slower, in this case the first one, is called the denser medium.
- When a ray of light enters a denser medium, it is
bent towards the normal.
- When a ray of light enters a rarer medium, it is
bent away from the normal.
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There
is an index of refraction (n) between the two mediums. To get a
value, we have to divide the sine of the angle in vacuum
or air by the sine of the angle in the denser medium. In the example above, the index of
refraction would be
n = sin a / sin b
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Some
of the light is always reflected. However, when a ray of
light goes from a denser medium to a rarer medium, all
the light will be reflected if the angle of incidence is
greater than the critical angle. The critical angle is the angle of incidence for which the
refracted ray is at 90 degrees with the normal. 
How do
lenses refract light?
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Section 6. Chapter 10 Quiz
Easy... All right. Try this quiz.
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