How Light Works

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• How Light Works
• Outline
• Light occupies a part of electromagnetic spectrum
• Wave and particle properties of light
• Interaction between light and objects
• Producing light
• Light is important to our eyes, as sound to our
  ears.
• Eye sees light from
• Devices that produce light, e.g. incandescent
  bulbs, fluorescent bulbs, the sun, and lasers,
  etc.
• Objects that reflect, or scatter, or transmit, or
  partially absorb light.

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• Light occupies a part of electromagnetic (EM)
  spectrum
• Like radio and microwave, light occupies a part
  of the EM spectrum
• (10nm 1mm, where 1nm 10-9 meter, 1mm 10-3
  m).

Radio Microwave
Both ultra-violet and infrared lights are
invisible.
Visible light Red (0.63 mm), Green (0.5 mm),
Blue (0.48 mm), where 1mm 10-6 m.
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Colors by addition Red Green Yellow Red
Blue Magenta Green Blue Cyan
White light (e.g. sun light) appears colorless,
but is made of many colors.
When white light passes through a prism, we see a
rainbow of colors (frequencies).
Colors by subtraction (absorption) The leaves of
green plants contain a pigment called
chlorophyll, which absorbs the blue and red
colors and reflects the green.
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• Wave and particle properties of light
• Light behaves like
• waves at longer wavelengths - light waves
• particles at shorter wavelengths photons
  (packets of energy)

Light behaves like waves
The waves are made up of energy. All waves are
traveling energy, and they are usually moving
through some medium, such as water for water
waves.
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But, light waves do not need a medium to travel
through they can travel through a vacuum. As
light waves travel through vacuum, their
electric fields are orthogonal to their magnetic
fields hence electromagnetic waves. c l
n where c the speed of light, l wavelength
and n frequency.

• c is measured in meters/second
• is the distance between any two corresponding
  points on successive
• waves, usually peak-to-peak or trough-to-trough,
  and is measured in
• meters
• n is the number of waves that pass a point in
  space during in a second.

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Interference is an important phenomenon that
comes from the wave nature of (coherent) light,
as shown in the following two experiments
In this experiment, light passes through two
small openings and spreads out as two spherical
waves. The two spherical waves interfere with
each other to give rise the interference pattern.
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Another interference experiment
When white light shines on a film with two
reflective surfaces, the reflected waves
interfere with each other to form rainbow
fringes. The fringes change colors when you
change the angle at which you look at the film,
because you are changing the path by which the
light must travel to reach your eyes.
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Light behaves also like particles, as illustrated
in the following two experiments The
photoelectric effect when ultraviolet light
rays (photons packets of energy hn) hits the
surface of a photoemissive material, electrons
are emitted from the surface.
Geometric optics light travels as rays in
straight lines and bounces off a mirror much like
a ball bouncing off a wall.
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Relation between light waves and rays
Light rays are always perpendicular to the
wavefronts of light waves.
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3. Interaction between light and objects When
Light Hits a solid Object What happens to it
depends on the energy of the light wave (hn) and
the energy levels of the material of solid
object. The energy levels of an isolated atom
and three generic solids with different
electrical properties are illustrated in the
figure below.
Conduction Band
Valance Band
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An energy level of an isolated atom corresponds
to an orbit of its outer electron. Higher
orbits correspond to higher energy levels. From
quantum mechanics, the energy levels of an
isolated atom are quantized. The lower energy
levels in the solids are similar to those of the
isolated atom, except that the energies of the
higher-lying discrete levels split into closely
spaced discrete levels and form bands. The
highest partially occupied band is called the
conduction band the valence band lies below it.
They are
separated by Eg, called the energy bandgap. The
lowest energy bands are filled first.
Conducting solids such as metals have a
partially filled conduction band at all
temperatures. The availability of many
unoccupied states in this band means that the
electrons can move about easily this gives rise
to the large conductivity in these materials.
Insulators have a filled valence band, mostly
filled conduction band and a large energy gap
between them. Few electrons can attain sufficient
thermal energy to contribute to the
conductivity. Semiconductors have energy
structures and properties between those of
conductors and insulators.
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When light hits metallic objects When light hits
some materials like metals (e.g. silver or
copper), electrons in the conduction band absorb
the photons and are energized. But, the
electrons send the light wave of the same
frequency as the incoming wave back out of the
material. The overall effect is that light does
not penetrate deeply into metals.
The angle of the reflected wave is equal to the
angle at which the incoming wave hits the
surface, q1 q2.

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When light hits insulator objects When light
hits insulating objects, it can pass through them
with no effect (transmission), or it can be
absorbed, reflected, refracted or scattered.
Many optical components made for visible light,
e.g. lens and prism, are made of glass. Glass is
an insulator, transparent to visible light, and
opaque to ultraviolet and infrared light.
Transmission occurs when the photons of visible
light do not have enough energy to overcome Eg
of the material. They are transmitted through
and not absorbed.
Absorption Photons of infrared light are
absorbed by glass they have only enough energy
to move the electrons in the conduction band
around within the band and heat up the material.
Photons of ultraviolet light are absorbed and
have enough energy to excite electrons in the
valence band to the conduction band. But, the
conduction band of an insulator is quite full
such that excited electrons cannot move around
freely to contribute to improved conductivity.
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Refraction When light hits a glass plate at an
angle, it will refract and reflect.
n1 sin q1 n2 sin q2 where n1 and n2 are the
index of refraction of the material outside or
inside of the object, q1 and q2 are measured
with respect to surface normal (the Snells
law). After passing through the first surface of
the glass plate, light will again be reflected
and refracted at the second surface. The angle
of the transmitted wave will be the same as that
of the incoming wave, but inside the material the
wave will be bended toward the normal of the
material surface.
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Scattering Scattering is merely reflection from
and refraction of a rough surface, e.g. a glass
shower door. Incoming light get reflected and
refracted at all sorts of angles because the
surface is uneven.
When light hits semiconductor objects Silicon
is an example of semiconductor material. It
absorbs visible light and is used to make
detector arrays for digital cameras. But, it is
transparent in infrared. Gallium arsenide is
another example of semiconductor material. It
is used to make many optoelectronic devices, e.g.
semiconductor laser.
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• Producing light
• Photons are emitted from atoms and molecules when
  they make transitions from
• an upper to a lower energy level. The frequency
  n or color of the photons can be
• related to the energy difference between the
  energy levels.
• D E hn, where h is the Plancks constant and n
  is frequency.
• There are many different ways to produce light.
• Heat to light most common, e.g.
• Incandescent light bulbs (use electricity to
  heat filaments)
• Halogen lamps (let the filaments run
  hotter)
• Electricity to light
• Fluorescent lights (use high voltage to ionize
  gases in vacuum tubes)
• chemicals to light
• Gas lanterns (use fuel like natural gas or
  kerosene as heat source)