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LIGHT INTRO
When an electron changes its energy level a photon of light
is either emitted or absorbed. A photon is the smallest "wave
packet" of light that can exist. Since moving electrons
are the chief moderators of energy-photon conversions, light
is a transverse wave composed of both electric and magnetic
field properties (electromagnetic waves). Light obeys the
wave equation from the previous tutorial, 
, but the velocity of light remains fairly
constant at about 3E8 (300,000,000) m/s. Light
was thought to have infinite speed up until 1675 when Olaf
Roemer used the eclipsing moons of Jupiter to predict that
it was less than 11 minutes per Astronomical Unit (AU = average
distance from the earth to the sun). Visible light (ROYGBIV
= Red, Orange,
Yellow, Green,
Blue, Indigo,
Violet) comprises a miniscule portion
of the electromagnetic spectrum. The rest of the spectrum includes
electric power waves, radio waves, short waves, microwaves, infrared
(before visible red), ultraviolet (after visible violet), x-rays,
and gamma rays.
Click Here for
an excellent spectrum demo
Just like sound energy, light energy that is emitted from
a source (equally in all directions) will decrease in intensity
with distance according to the inverse square law. The amount
of visible light that falls an a surface is called
luminous Flux (unit:lumen). 1 lumen equals
the amount of light that falls on each square meter of
a sphere surface with a radius of 1 meter and a
light source at the center with an Intensity of 1
candela.To figure the total luminous Flux that
a source of a given Intensity has use F = 4 I
. If the light is not hitting the surface straight on
(perpendicular/normal) then the intensity of the luminous
flux is reduced according to the angle of incidence. I'
= I cos .
This accounts for how the tilt of the earth to the sun affects
the seasons.
REFLECTION, REFRACTION, DIFFRACTION
Light can be reflected and refracted. Reflection
occurs when the photons of light are absorbed and then emitted
in the opposite direction in which they came. Refraction
occurs when a transparent object bends the path of the photons
as they pass through the medium. Rays from an object (at
a distance do ) hit the surface of a curved mirror
or lens and are then reflected/refracted past the focal
point ( f ) and may converge to form an image
if a screen is placed at a specific distance di , and
follow the equation f = 1 / (1/do + 1/di). Transparent
materials will bend light more or less according to their
index of refraction (n). Air has a reference index
of n = 1 and water n = 1.33. The incident angle i
and its index of refraction follow an inverse relationship
with the refracted angle r
and its index of refraction.
Click Here for a nifty lens refraction demo!
Light rays can also be bent when they pass near and object
as well. This is called diffraction. If a hair with
diameter do is placed in the path of a laser beam of
wavelength 
it will demonstrate wavelike properties by making an unexpected
interference pattern on a screen a distance L from
the hair. The distance the interference spots are apart d
form a geometric relationship that can be exploited to find
the thickness of the hair causing the diffraction pattern.
Ask the teacher for the laser and perform this experiment
in class!
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